In situ MAS NMR spectroscopy under flow conditions and on-line gas chromatography have been applied to study the onset of the conversion of methanol on zeolite HZSM-5 at temperatures between 373 and 573 K. In the steady states of methanol conversion at T greater than or equal to 523 K, by on-line gas chromatography mainly the formation of ethene and propene was observed. Simultaneously recorded in situ C-13 MAS NMR spectra show signals at 12-25 ppm and at ca. 125-131 ppm indicating the presence of adsorbed C-4-C-6 olefins. The observation of these adsorbates on a working catalyst supports the "hydrocarbon pool" mechanism previously proposed for the methanol-to-hydrocarbon conversion on acidic zeolites. Methanol conversion at 473 and 573 K and subsequent purging of the catalyst with dry nitrogen at 293 K led to a C-13 MAS NMR signal at 59 ppm due to methoxy groups. No hints to the presence of ethoxy, propoxy or butoxy groups and the formation of alkyl oxonium ions were found by in situ C-13 MAS NMR spectroscopy under flow conditions.