An operating method for a system including a reformer which in reformate mode can convert hydrocarbon-containing fuel to a hydrogen-rich reformate gas, and a catalytic exhaust gas aftertreatment device which is acted upon by reformer gas to reach the catalytic converter light-off temperature more rapidly. During cold starts, the reformer, after its own starting phase, is initially operated in a so-called lean-burn mode, and the reformer is switched from lean-burn mode to reformate mode as soon as combustible constituents in the catalytic aftertreatment device can be independently oxidized. During the reformer lean-burn mode, the combustible constituents of the engine's exhaust gas react with the hot reformer gas, such that the cumulative exhaust gas flow has the composition of exhaust gas generated by stoichiometric combustion.