Results of comparative testing of the LPG (Liquefied Petroleum Gas) fuel and such a fuel with an addition of light aliphatic hydrocarbons, chiefly methane, in two versions have been presented. The tests were carried out in respect of the physicochemical properties of the fuels according to Standard PN-EN 589 and in respect of the performance characteristics of a motor vehicle powered with the fuels under test. The main purpose of the tests was to determine the acceptable fraction of light aliphatic hydrocarbons, predominantly methane, in the LPG fuel, whose basic components are C3 and C4 alkanes or alkenes. The addition of light aliphatic hydrocarbons to LPG may be of considerable importance for the fuel properties, primarily because of high values of the saturated vapour pressure of such admixtures. The fuel composition was determined with the use of the gas chromatography method. The methane additive content of the fuels under test was 0.1% m/m7 and 0.2% m/m and the ethane additive content was 0.2% m/m. The fuels met the requirements of Standard PN-EN 589. The performance of the motor vehicle was tested on a chassis dynamometer. No adverse effect of the addition of light aliphatic hydrocarbons to the LPG fuel on vehicle engine’s effective power and pollutant emissions was revealed.