Special depreciation and investment allowances were introduced to stimulate the capital outlays of firms. In this investigation we determine the extent to which these allowances actually influenced the timing or amount of capital expenditure undertaken by fifty-five companies which participated in a questionnaire survey. These companies accounted for about thirty per cent of the capital' expenditure on plant and machinery undertaken in 1970-71 by establishments analysed in Statistics of Industrial Production. The annual accounts of the above companies and those of fifty-six other companies~ were also examined. From the 1963-64 income year, a company was permitted to claim special depreciation without charging a similar amount. Prior to 1969, no more than ten per cent of the companies whose· accounts were examined were known to take advantage of this ruling. From the 1968-69 income year, a company was also permitted to claim ordinary depreciation without charging a similar amount. This change in taxation practice has enabled companies to compute their depreciation charges on plant and machinery on the straight-line method. In 1973, eighteen per cent of the companies whose accounts were examined used this method. All of the companies which disclosed in their annual accounts the method used to account for investment allowance reflected the tax saving arising from claiming that allowance in a lower taxation charge in the year in which investment allowance was claimed. Given realistic assumptions and values for relevant variables, special depreciation increased the net present value of capital expenditure proposals by between 1.7 and 4.5 per cent of the cost of the qualifying asset. In respect of the year ended 31 December 1971, 9.2 per cent of the participants in the questionnaire survey answered that special depreciation resulted in the acquisition of qualifying assets at an earlier time than would have been the case if that allowance had not been available. Seventeen per cent of the companies-gave a similar answer in respect of investment allowance. Firms which were responsive to the timing effect of special depreciation and investment allowance accounted for less than five and eleven per cent respectively of the capital expenditure on plant and machinery undertaken by participants. If special depreciation or investment allowance had been abolished or suspended for a period of three to four years from 31 December 1970, it is estimated that the value of minor capital expenditure projects approved by participants in 1971 would have been two per cent less than actual approvals. In addition, three firms would have delayed or abandoned major capital expenditure projects. Participants generally used capital expenditure decision techniques which were less sensitive than discounted cash flow methods to special depreciation and investment allowances. Firms which answered that the most significant constraint on further capital expenditure was an inadequate rate of return on proposals considered, and fast-growth companies, were more responsive to the timing effect of special deprecation than were other firms. There were no other statistically significant relationships between firms' responsiveness to the abolition, suspension or timing effect of special depreciation or investment allowance and the large number of variables considered. On the basis of the results presented herein, it is concluded that the timing and amount of capital expenditure undertaken by participants in the questionnaire survey were influenced to a very minor extent by special depreciation and investment allowances.