A theoretical study of the nitration of benzene by acyl nitrates catalyzed by protonated zeolites is reported. The calculations were performed at the DFT/B3LYP/6-31G** level, using a 5T cluster as a model for the acid site. Contrary to what was previously suggested, the nitrating agent is a nitronium-like species. In spite of that, the reaction follows a different mechanism from the one in a solution of HNO3 and H2SO4. The estimated activation barrier is of 19.3 kcalmol(-1) and the reaction is exothermic. (C) 2004 Elsevier B.V. All rights reserved.