Adopting a philosophical perspective, this article examines the implications of both formal and substantive gender equality for the design of a gender-responsive personal income tax (PIT) system. Analysing several family-related PIT provisions, this article discusses which provisions create structural inequalities by producing indirect gender discrimination. The article demonstrates that a formal approach to gender equality is insufficient for remedying the structural disadvantages women face by tax systems. Formal equality does not sufficiently recognize the problem of indirect gender discrimination in taxation. Substantive interpretations of gender equality, on the other hand, counteract both structural disadvantages and gendered role divisions produced by PIT provisions. Accordingly, substantive gender equality rejects tax provisions that produce indirect gender discrimination and, therefore, provides a more pervasive approach to equality than formal interpretations do.