Sustainable economic growth in Poland requires a high level of innovation. At the moment, innovation in the Polish economy is low due to factors such as insufficient research and development (R&D) expenditure in the corporate sector. In 2006, tax incentives were introduced to stimulate R&D in enterprises. However, they failed to produce the expected results. The incentives enjoyed little interest among businesspeople, and in consequence failed to contribute to increased spending on R&D in the corporate sector. The paper aims to evaluate these tax incentives by using international comparisons and putting forward proposals for changes in the tax break system. The existing system was examined with the use of the B-index (“before-tax income needed to break even on one dollar of R&D spending”), a popular measure of the tax system’s influence on investment in R&D applied by the Organization for Economic Cooperation and Development (OECD). The B-index is based on the idea of a marginal effective tax rate. It measures the relative profitability of R&D expenditure in a given tax system. The analyses made by the author show that the Polish tax system has a negative impact on R&D in the corporate sector—more detrimental than suggested by the B-indexes calculated by the OECD. The results obtained lead the author to conclude that the tax regulations currently in force in Poland are among the least favorable among OECD countries in terms of their influence on R&D in the corporate sector. According to Adamczyk, the main reason behind the unfavorable influence of the Polish tax system on the profitability of R&D in the corporate sector is that the existing tax breaks apply to a limited number of taxpayers. Other causes include inadequate income tax rates and tax amortization regulations used in the country. Poland’s current tax instruments designed to support R&D, due to their highly selective nature, contradict the idea of direct fiscal incentives, Adamczyk says. He adds that the effectiveness of tax breaks may also depend on factors not covered by the B-index, such as the transparency and stability of tax regulations.