Pre-print, Working paper: This paper looks whether the choice to contract a civil union is motivated by fiscal reasons. Since 1999, in France, different-sex couples can contract either a marriage or a civil union, called a pacs. It is a new legal form of union, more flexible than marriage. The pacs turned to be successful, 30% of new unions in 2007 were pacs. In 2005, the income taxation of pacsed partners changed to benefit more to pacsed couples. This paper investigates the link between the reform of the income taxation and the growing number of pacs by analyzing the impact that should be observed if couples answer was the optimal one. It shows that about 38% of the newly contracted pacs since 2005 can be attributed to the reform. This tends to show that the choice to contract a pacs is determined by short-term views. Then it questions the link between marriage and civil union, the pacs could be either thought as a substitute or a complementary to the marriage.