Pre-print, Working paper: Focusing on two different mass privatization schemes in two transition economies, Poland and the Czech Republic, we show that the ownership structure in the two countries has rapidly evolved since the initial distribution of property rights Ownership concentration has significantly increased and we can observe an important reallocation of ownership claims between different groups of shareholders. This evidence goes against the main argument of the critics of mass privatization concerned with the dispersed ownership structure these programs were supposed to generate. The fact that the degree of ownership concentration is similar in Poland and in the Czech Republic suggests that private benefits of control are large in both countries. However, when we consider the determinants of ownership concentration we find an interesting difference: in the Czech Republic the increase in ownership concentration is less likely in poorly performing firms while in Poland the quality of past performance does not affect investors' willingness to increase their holdings. This contrasting effect may be interpreted in the light of the theory stressing the importance of the quality of the legal system for investors' behaviour: Poland is usually praised for high standards of its regulation while the Czech Republic, especially in the early and mid-1990s, has been blamed for its weaknesses. So, although direct comparison of ownership concentration in the two countries does not provide confirmation of the main prediction of "law matters" theory, we find indirect evidence in its favour.