This paper develops a model to explain why in some economies, and especially in the Brazilian economy, we see evidence that taxation is not smoothened when facing real interest rate shocks. Instead, tax rates vary following the economic cycle. We show through a formal model the connection between fiscal sustainability and the variation of the level of taxation, a pattern more often observed in countries that went through fiscal adjustment plans - a situation that might suggest the possibility of occurrence of confidence crisis regarding the payment of sovereign debt, or a positive probability of debt default. From the model, it is possible to conclude that a non-constant taxation path can be established as an optimal result in such economies. It is a model that allows choosing endogenously and optimally the necessary fiscal superavit in each period of time.