The aim of this paper is to describe and discuss the evolution of corporate tax design in Italy. More in detail, an assessment of the Allowance for Corporate Equity mechanism (introduced in 1998, stopped in 2004 and reintroduced in 2011) is performed through a microsimulation model for the period 1998-2008. Results show that thehigh instability of the corporation tax framework did not succeed in lowering the fiscal burden; on the contrary, it hampered investmentsand can be considered highly inefficient.