The stable isotopic composition of plant tissue reflects the biophysical and biochemical processes that occur during water uptake and cellulose synthesis. Stable isotope techniques in plant science have primarily focused on vascular plants; however, there is a growing interest in the stable isotopic composition of nonvascular plants, especially for paleoclimate reconstruction. The stable carbon isotopic composition of plant organic matter provides information of when a plant was photosynthetically active, while the stable oxygen isotopic composition can infer the environmental conditions that favoured photosynthesis. Presently, little is known about the relationship between the stable oxygen isotopic composition of moss tissue and the environment due to the complexity of the isotopic fractionation processes involved. This thesis offers insight on the oxygen isotopic composition of moss tissue and phyllid water through three separate studies and addresses the possible isotopic fractionation processes that may influence the stable isotopic composition observed. The first study investigated the oxygen isotopic composition of evaporated water from two species of moss: Ptychomnion aciculare and Dawsonia superba. Measurements of on-line gas exchange and the isotopic composition of vapour water from the mosses were taken. Two oxygen isotope theories were applied to explain the observed oxygen isotopic composition of evaporated water. The first isotope theory was Rayleigh’s distillation theory, often used to describe the oxygen isotope variation observed in the global hydrological cycle. The second theory was the modified Craig–Gordon model, often applied to isotopic enrichment in vascular leaves. The results highlighted how the anatomical differences of the phyllid structure influences the isotopic composition of evaporated and transpired water. The second study interpreted the stable carbon and oxygen isotopic composition of D. superba stems. This involved taking 10 mm transverse sections along the stem and analyzing the isotopic composition of each section. The results from the carbon isotope analysis indicated that the carbon isotopic composition was more depleted at the base of stem compared to the tip, suggesting soil respiration, rather than atmospheric CO₂, was the primary factor influencing the isotopic composition of the stem. Additionally, analyses of the stable oxygen isotopic composition of D. superba stem indicated seasonal variations that coincided with the seasonal shifts in the oxygen stable isotopic composition of precipitation. The final study investigated the relationship between the isotopic composition of Sphagnum cuspidatum bulk tissue and the oxygen isotopic composition of source water was investigated under laboratory settings. The oxygen isotopic composition of moss tissue is theorized to reflect the oxygen isotopic composition of source water, offset by the biochemical fractionation constant. Four sealed growth chambers were set up and the mosses were watered with different isotopically labelled water for 6 months. S. cuspidatum was harvested, dried, and analyzed for the stable isotopic composition. The results indicated that oxygen isotopic composition of moss tissue increased by 0.17‰ for every 1‰ increase in source water, a relationship much lower than predicted by the theory. This suggested that the isotopic composition of moss tissue reflects an equilibrium process between source water and the vapour from atmospheric air. These studies contribute theoretical and practical advancements in understanding the relationship between moss stable isotopes and the environment. The research here demonstrated the relationship between the oxygen isotopic composition of moss bulk tissue to the oxygen isotopic composition of precipitation and water sources, in both field and laboratory settings. They also provide insight into how the physiology and anatomy of moss phyllids influence the observed isotopic composition fixed in organic material.