One of the main objectives to be accomplished by the European Union law is to eliminate barriers to the functioning of domestic market and in particular improve the competitiveness of enterprises. After several years of efforts, on 16 March 2011 the European Commission approved a proposal for the directive on a Common Consolidated Corporate Tax Base which is to remove obstacles to the functioning of internal market and increase tax harmonization in the EU. The paper addresses issues relating to tax in corporate finance. Canons of taxation will be discussed and special emphasis will be placed on principles behind formulating fiscal law provisions (including the EU law). Furthermore, the article presents the results of surveys into the importance of taxation canons for Polish and EU companies.