This paper presents a simplified application of the collective model of labour supply. To identify the model, some preference parameters are estimated, while some other key parameters are calibrated so as to replicate as best as possible observed labour supplies under the 1994 personal tax system in Spain. The model is used to simulate the reform of the tax system introduced in 1999. It is seen that the reform induces important behavioral changes. It leads to a decrease in average tax rates, a larger redistributive effect on the before tax income distribution than the 1994 tax system, and an increase in social welfare defined on the after tax, or disposable income distribution.