In a 2-stroke cycle crankcase-scavenged gasoline engine, the compositions of cylinder gas and exhaust gas were investigated by analysis of the sampled gas extracted from the cylinder and exhaust pipe. As is well known, the combustion process taking place in a 2-stroke cycle gasoline engine has cycle-by-cycle variations. In this study, a new method controlling the operation of the sampling valve was applied, which enabled us to select cycles showing the same peak pressure and to extract a combustion gas produced by the similar combustion process.From these data the following were confirmed: 1. For estimating the composition of combustion gases, simple equations of combustion reactions are available. 2. The most reliable values estimating the scavenging properties may be calculated from the change of O2 concentrations of sampled gases before and after scavenging. 3. It is necessary to select the gas sampling cycle by the cylinder pressure, because the scavenging process is affected by the combustion process in the preceding cycle. 4. No remarkable uneven local composition is observed at the time of spark ignition.