The state of the acylating agent acetyl chloride, adsorbed on a series of proton and metal ion exchanged zeolites X, Y (faujasite) and ZSM-5, was investigated by C-13 MAS NMR spectroscopy. The observed carbonyl signals were assigned to two species : chemisorbed acetyl chloride bound to lattice oxygen (signals near 182 ppm), and acetyl chloride complexed with counter cations in the lattice (signals near 172 ppm). In a few cases signals were observed which have been assigned to "free" acylium cation stabilized on the surface of the solid (signal 160-165 ppm). Experiments in which toluene was adsorbed on to ZnY pretreated with acetyl chloride showed the participation of various adsorbed species in the acylation reaction of toluene.