The investigations on an indirect injection unsupercharged engine fitted with a two state oxygen sensor have shown a particularly high fluctuation of mixture composition in transient states of the engine, represented by the rev up and rev down of the engine without a load and with a steady load. The spread of the maximum and average cycle by cycle mixture compositions in these states determined by a method developed by the authors (this method shall be a subject of a separate publication) is larger if the engine load is lower. It has been proved that the biggest fluctuation of the mixture composition occurs in the first part of the rev down phase i.e. in the phase of stepwise throttle closing when the injectors are still activated. In steady states represented by typical operating states of the engine the highest fluctuation of the mixture composition occurs at high engine speeds and low engine loads, and the lowest at low engine speeds and high engine loads. It has been confirmed that the fluctuation of the cycle by cycle mixture composition in the transient states is almost one order of magnitude larger than in the steady states. The investigations have been performed on a 1.6l Euro 3 engine fitted with a control unit made by a reputable manufacturer.