Abstract: A process and a system are used for controlling the mixture composition for an Otto engine with an NOx storage catalyst during a regeneration phase. The process includes controlling, to the value of one, a &lgr;-value (&lgr;M) of an exhaust gas fed to the NOx storage catalyst during a regeneration phase as a function of a &lgr;-value (&lgr;K) of an exhaust gas emitted by the NOx storage catalyst. The system includes a control device for controlling, to the value of one, a &lgr;-value (&lgr;M) of the exhaust gas fed to the NOx storage catalyst during the regeneration phase as a function of the &lgr;-value (&lgr;K) of the exhaust gas emitted by the NOx storage catalyst.