Major differences between the nominal tax rate and the effective tax rate on corporate income are found in countries with stable taxation systems. The study’s aim was to examine the relationships between both rates in Poland. The marginal effective tax rate concept based on the cost-of-capital approach developed by R. Hall and D. Jorgenson can be taken for a measure of that relationship. The level of the marginal effective corporate tax rate may be affected by both microeconomic factors, such as the utilization structure of production assets or their funding structure, and macroeconomic variables, such as interest rates and tax policy instruments, among which accelerated depreciation and so-called investment tax incentives have been analysed in the study. The obtained results confirm the hypothesis that there were substantial differences between the nominal tax rate and the effective tax rate on corporate incomes in Poland in the 1992-2000 period, which even reached the marginal effective tax rate level. Nevertheless, with the results of calculations related to the level of investment outlays, no relationship is found between the level of the marginal effective tax rate and that of investment spending, which suggests that the mentioned tax incentives were ineffective and that tax factors were not of a serious relevance for investment decisions. This finding becomes less substantive when related to accelerated depreciation. It seems, however, that this instrument does not perform a stimulating function and is used by its addressees only to reduce the tax burden, especially when this does not involve a necessity to fulfil requirements which would enforce change of the adopted action strategy.