The planned reform of fiscal equalisation among Swiss cantons is partly based on the Aggregated Tax Base (ATB), which represents the fiscal capacity of different cantons. As some types of companies are favourably taxed, the profits of these companies cannot completely be added to the Aggregated Tax Base. The scope of this project was to estimate the extent to which particular profits of holding companies, domicile companies, and mixed companies can be added to the measure of the ATB. In the first step, we developed criteria to determine whether a particular cantonal source of fiscal revenue should be considered in calculating the ATB. In the second step, we applied these criteria to the relevant cantonal tax rules. Finally, we quantified the factors in question. The quantification was based primarily on the results of a questionnaire that was sent to all 26 cantonal tax departments and on theoretical considerations on tax competition and the economic effects of fiscal equalisation.