This paper presents the results of measurements conducted to determine particle and gas emissions from two large compressed natural gas (CNG) spark ignition (SI) engines. Particle size distributions in the range from 0.01-30 mu m, and gas composition were measured for five power settings of the engines: 35, 50, 65, 80 and 100% of full power, Particle emissions in the size range between 0.5 and 30 mu m, measured by the aerodynamic particle sizer (APS), were very low at a level below two particles cm(-3). These concentrations were comparable with average ambient concentration, and were not considered in the succeeding analysis. Both engines produce significant amounts of particles in the size range between 0.015 and 0.7 mu m, measured by the scanning mobility particle sizer (SMPS). Maximum number of concentrations of about 1 x 10(7) particles cm(-3) were very similar for both engines, The CMDs were in the range between 0.020 and 0.060 mu m. The observed levels of particulate emission are in terms of number of the same order as emissions from heavy duty diesel engines (Morawska et (al., Environ. Sci. Tech. 32, 2033-2042). On the other hand, emissions of CO and NOx of 5.53 and 3.33 g kW h(-1), respectively, for one of the tested engines, were considerably lower than set by the standards. According to the specifications for the gas emissions, provided by the US EPA (US EPA, 1997), this engine can be considered as a "low-emission" engine, although emissions of submicrometer particles are of the same order as heavy-duty diesel vehicles. (C) 2000 Elsevier Science Ltd. All rights reserved.