The main objectives of this work were to find out: (a) how the stable isotope ratios in wood constituents such as cellulose and lignin vary with changing climate; (b) whether such variations can be applied to the study of past climate. Initially rings from a New Zealand grown Pinus radiata (Monterey pine) were studied since this is a complacent tree which grows throughout the year and hence over a wide temperature range. In order to accurately determine the stable isotope ratios in wood components the following techniques were developed: (a) the preparation of wood constituents on a small scale; (b) the determination of δD values in waters; (c) the determination of δD values in plant materials; (d) the determination of δ¹³C values in plant materials. Several possible methods for measuring D/H ratios for cellulose are discussed. It was decided to use a technique which involves heating cellulose with a standard water to fix the D/H ratio of the exchangeable hydrogens. Using this method relative δD values have been determined to a precision of ± 0.9°/οο (1 σ). The δ¹³C method was developed so that the ¹³C content of cellulose and lignin samples could be determined to ± 0. 04°/οο (1 σ). A new procedure for determining δ values from raw mass spectrometer results is outlined. δD determinations on cellulose samples from three growth rings in a specimen of Pinus radiata show that the non-exchangeable hydrogens of cellulose are more than 20°/οο depleted in the summer as compared with the winter. These δD values appear to vary with the annual temperature cycle, but in the opposite sense to that expected from thermodynamic considerations. δ¹⁸ο determinations on the same samples show a summer/winter variation of about 2°/οο and in the opposite direction to the summer/winter variation in the δD determinations. The δD results from Pinus radiata cellulose are compared with δD determinations on cellulose from other trees. It is concluded that all the measurements to date suggest that an increase in temperature results in a decrease in the D/H ratio of the non-exchangeable hydrogens in cellulose. The results imply a temperature coefficient of about - 3°/oo per °C, which is opposite to that found for precipitation (+ 5. 6°/oo per °C). This temperature effect will only be useful for past climate studies if δD variations due to meteoric and/or leaf transpiration changes can be corrected for. This may be possible using δ¹⁸o determinations. A new model to explain the effects of environmental changes on the magnitude of carbon isotopic fractionation in C₃ plants is presented and discussed. δ¹³C determinations on both cellulose and lignin from the same Pinus radiata that was used for the δD determinations show an enrichment of about 2°/oo in summer wood as compared with winter wood. This summer/winter variation is probably mainly due to changes in air temperature affecting the magnitude of carbon isotope fractionation during photosynthesis. The lignin variations are similar, but significantly displaced in time with respect to the cellulose variations. This is probably the result of secondary lignification. δ¹³C values for cellulose prepared from individual rings in a sample of dendrochronologically dated Pseudotsuga menziesii (Douglas fir) are compared with meteorological observations during growth. A correlation between the δ¹³C values and mean late spring/early summer temperature is discussed. The temperature coefficient for this correlation is + 0.22°/oo per °C, which is in good agreement with the Pinus radiata results. Similar determinations on individual rings from Pinus radiata are difficult to interpret in terms of climatic changes because of a relatively large depletion in ¹³C towards the centre of the tree studied. A similar depletion in ¹³C was observed for cellulose prepared from a carbon-14 dated Agathis australis (New Zealand kauri) which grew during the last 1000 years. However, no such “age” effect was observed when δ¹³C values were determined for cellulose samples prepared from two dendrochronologically dated Pinus longaeva (Bristlecone pine) trees which also grew during the last 1000 years. The short-term fluctuations in the cellulose δ¹³C curve for Agathis australis are probably temperature related, and therefore represent a record of warm and cold periods in New Zealand’s climate. This record is discussed together with information available from other sources. It is concluded that the fluctuations in New Zealand’s climate during the last millennium were basically similar to those in the climate of North-western Europe. The δ¹³C variations in Pinus longaeva cellulose appear to be climate related, although in the opposite sense to the Agathis australis variations. These δ¹³C variations in lower forest border Pinus longaeva significantly correlate with upper tree line ring width variations for the same species. It is proposed that changing water stress is the underlying cause of the δ¹³C variations. If this is the case, the correlation may be the result of a correspondence between periods of water stress at the lower forest border and warm periods at the upper tree line. It is concluded that ¹³C/¹²C ratios in both cellulose and lignin vary with changing climate. Results from Pinus radiata and Pseudotsuga menziesii indicate a temperature dependence of about + 0.21°/oo per °C. However, other results from Pseudotsuga menziesii and Pinus longaeva indicate that, in some situations, this effect is dominated by a larger and opposite effect, attributed in this work to water stress. Thus there is little doubt that δ¹³C variations in the wood components from each tree ring sequence studied reflect climatic changes in that particular area. For some trees the effect of plant physiological changes on such variations is little understood. At present, those trees growing in exposed situations with a plentiful water supply appear to hold most promise for elucidating past climate data from measurements of ¹³C/¹²C ratios.