New experimental techniques of in situ MAS NMR spectroscopy introduced in the past decade made this analytical method to a useful toot for the study of heterogeneously catalyzed reactions. Batch and flow techniques were applied to study reactions catalyzed by basic and acidic solids. The present review introduces into in situ MAS NMR investigations of transition states, e.g., via the HID exchange between acidic surface OH groups and reactants, the alkylation of aromatic compounds (toluene, aniline), and the formation of light olefins by conversion of methanol (MTO) on acidic zeolites. The studies of the MTO process clearly demonstrate the advantages of modern continuous-flow (CF) and stopped-flow (SF) approaches of in situ MAS NMR spectroscopy in heterogeneous catalysis. (C) 2004 Elsevier B.V. All rights reserved.