Low sulfur oxides concentration have been used to activate monolithic catalysts at different activation temperatures and various SO2/SO3 inlet ratios in order to evaluate the feasibility of their activation with flue gas. In these conditions, the activation of the catalyst does not depend on the SO2/SO3 feed ratio; however high temperatures and activation times over 50 h are required. The activation of the catalyst carried out at low SOx concentrations slightly improve the final activity as compared with those obtained by the conventional activation step where higher SOx and O-2 concentrations are used.