Abstract: The invention relates to a process for controlling the fuel-air ratio for a gas engine which is provided with one or more .lambda. (1) sensors and a three-way catalyst, as well as a control system for adjusting the fuel-air ratio by means of a control element on the basis of the signal of a first .lambda. (1) sensor, comprising stepwise adjusting the .lambda. (1) value at regulated intervals, at constant load of the engine, and measuring the corresponding activity of the catalyst, determining the corresponding measuring signals of the .lambda. (1) sensor, and determining the value of the measuring signal at which the activity of the catalyst considerably increases or decreases, followed by adjusting the value of the control signal of the .lambda. (1) sensor which is maintained as the desired control value on the basis of the latter measuring signal.