ALT To collect income taxes, almost all countries require employers to withhold monthly tax prepayments which are then fully credited against the final income tax liabilities of their employees. Despite being a fundamental component of income taxation systems worldwide, the impact of these withholding taxes on labor supply is poorly understood. We investigate their importance in the context of married couples in Germany where the withholding tax liability can be redistributed between spouses. We exploit a reform that reduced the withholding tax for some married women more than for others, while inducing no differences in income taxes. Using administrative data for the full population of German taxpayers, we estimate an elasticity of labor income with respect to the withholding tax eight years after the reform of 0.14. Additional evidence from a self-conducted survey suggests imperfect understanding of the tax system and limited pooling of resources within the household as the main mechanis