Investigating the formation of methyl-tert-butyl ether (MTBE) on zeolites HY, HBeta, HBeta/F and HZSM-5 by in situ C-13 MAS NMR spectroscopy under flow conditions, signals of alkoxy species have been observed at 77-90 ppm. On-line gas chromatographic analysis of the reactant and product molecules leaving the MAS NMR rotor reactor yielded that the formation of these alkoxy species is in line with high yields of MTBE and vice versa. Methanol adsorption experiments performed by in situ H-1 MAS NMR spectroscopy under flow conditions indicated that methanol molecules adsorbed on the highly active zeolite HBeta are characterized by a low protonation degree. Based on the experimental results of in situ H-1 MAS NMR spectroscopy and on-line gas chromatography, a mechanism of the synthesis of MTBE on acidic zeolites is proposed in which the reaction of isobutene with bridging OH groups leading to the formation of isobutoxy species with 'carbenium-like properties' plays a key role.