This article proposes an econometric approach to the estimation of changes in the CIT gap as well as to their decomposition into cyclical and structural components. The method was used to estimate changes in the CIT gap in Poland in years 2016 through 2018. The obtained results indicate that around 40 percent of an increase in CIT revenues over this period (i.e. 6 billion PLN) was of structural nature, i.e. cannot be explained either by the growth in the tax base, or the phase of the business cycle. Due to the lack of any major changes in CIT system over this period of time, this increase can be attributed to improved efficiency of tax collection.