How great an effect does the structure of income taxes have on women's labour market participation? This issue is investigated using a discrete choice static labour supply model for married couples in Ireland. The model incorporates fixed costs of working and simultaneously explains participation decisions and preferred hours of work. Details of the tax system are fully incorporated, and key elements of the welfare system are also taken into account. The model is estimated using data from the 1994 wave of the Living in Ireland Survey. The results are used to analyse the labour supply effects of a move to greater independence in the tax treatment of couples. The influence of tax structure on participation is reconsidered in the light of trends in women's participation in the labour market and two key changes in the structure of taxation: a shift from a joint or aggregated basis of assessment to an 'income-splitting' system in 1980 and a further substantial shift from income-splitting towards greater independence from 2000 onwards.