The study evaluates the relevance and validity of certain tax instruments used in economic practice. This involves instruments such as the possibility of selecting the form of corporate income taxation; more convenient dates and forms of advance tax payments; technology-related tax breaks; and tax-risk reduction instruments such as binding tax interpretations and advance pricing agreements (APA). The analysis was made on the basis of data from three tax offices in Poland. The author reached the following conclusions: 1) changes in eligibility criteria for using tax instruments are inconsistent with the rules of tax certainty and the fact that tax collection must be as inexpensive and efficient as possible; 2) more convenient advance tax payment deadlines do not prove to be useful because taxpayers tend to take little interest in them; 3) legal regulations accompanying tax privileges tend to reduce the effectiveness of these privileges in stimulating economic processes. The number and variety of instruments included in the Polish tax system testify to its strong stimulating function but also its complexity, Zbroińska says. Changes in the tax law system designed to either introduce or withdraw individual instruments lead to the destabilization of business activity by infringing on the rule of tax certainty. Some instruments fail to produce the desired results because access to them is restricted. Examples include plans to restrict the use of flat tax rates, limit access to the new-technology tax break, and impose steep fees on advance pricing agreements. The uncertainty among businesses leads to their marginal use of tax facilities related to advance income tax payments, the author says.