We use data from staff logs (Quadros de Pessoal) to discuss different procedures to decompose gender wage differential and examine the reasons underlying the evolution of the gender wage gap for the period 1985-1997 in the Portuguese labor market. Our results show that, for the first period, the increase of changes in the wage gap is mostly due to the increase in wage discrimination by means of the males´ wage advantage and of the females´ wage disadvantage. Growing inequality within gender groups, namely the rapid increase in educational attainment of women, and their probable overqualification in many jobs, may explain these results. Furthermore, the process of integration in the European Community responsible for deep changes in the economy, has also to be taken into account. The impact of the above facts was more moderate in the period 1991-1997, explaining the decrease of changes in the gender wage gap and the lower dispersion of the different components contribution to the relative wage gap.