The catalytic role of the hydrocarbon pool formed during the conversion of methanol on zeolite H-ZSM-5 was investigated by in situ MAS NMR spectroscopy under continuous-flow (CF) conditions and by changing of the methanol feed from (CH3OH)-C-13 to (CH3OH)-C-12 under steady state conditions. Utilizing this experimental approach, a decrease of the C-13 CF MAS NMR signals of alkyl groups bound at the olefinic and aromatic skeleton atoms of the hydrocarbon pool by 40% was observed. Simultaneously performed H-1 CF MAS NMR measurements ensured that the total amount of carboneous compounds on the zeolite catalyst was constant during the experiments. The decrease of the number of C-13-isotopes of alkyl groups, observed after changing the C-13-enrichment of the methanol feed, is the first direct evidence for the side-chain methylation of olefinic and aromatic skeleton carbon atoms as an important pathway of the hydrocarbon pool mechanism for the methanol-to-olefin conversion on acidic zeolites.