From experiments in a chromatographic reactor, under conditions of incipient reaction, the effect of the Si/Al ratio of H-ZSMS zeolites on the nature of the primary products has been proven in the transformation of methanol into gasoline. It has been determined that alkenes (ethene, propene and butenes) are the first reaction products, the proportion of heavier alkenes being greater as the Bronsted/Lewis ratio increases and as total acidity decreases (decrease in acidic site density). By in-situ FTIR analysis of the adsorbed phase, the presence of surface methoxy groups has been determined. By virtue of the results, a mechanism of propagation-decomposition of oxonium ions has been proposed. This mechanism basically occurs in a phase adsorbed on the zeolite and explains the formation of the first C-C bond and the presence in the gas phase of the first products according to a reaction-diffusion compromise for each one of the intermediate products of the mechanism.