Process and apparatus for methanol reforming in producing hydrogen gas for electric vehicle fuel cells in which the gas mixture to be reformed is passed through a catalyst-containing reaction compartment. The active length and/or the active inlet cross-section of an intake-side reaction compartment section which is temperature-controlled for high methanol conversion can be set as a function of the throughput of gas mixture to be reformed. Thereby, an essentially constant residence period of the gas mixture to be reformed results in the reaction compartment section which is temperature-controlled for high methanol conversion. The methanol reforming may thus be carried out even in the case of markedly fluctuating throughputs of gas mixture to be reformed with a constant level of methanol conversion rate and constantly low formation of undesirable carbon monoxide.