The Canadian federal tax reform of 1988 replaced a spousal tax exemption with a non-refundable tax credit. This reduced the'jointness'of the tax system: after the reform, secondary earners'effective'first dollar'marginal tax rates no longer depended on the marginal tax rates of their spouses. In practice, the effective'first dollar'marginal tax rates faced by women with high-income husbands were particularly reduced. Using difference-indifference estimators, we find a significant increase in labour force participation among women married to higher-income husbands. Copyright 2007 Institute for Fiscal Studies.