Examination is made of the operational limits in two variable compression-ratio single-cylinder engines when operating on the gaseous fuels methane, propane, LPC, and hydrogen under a wide range of conditions. Two definitions for the limits were employed. The first was associated with the first detectable misfire on leaning the mixture, while the second was the first detectable firing under motoring condition in the presence of a spark when the mixture was being enriched slowly. Attempts were also made to relate these limits to the corresponding values for quiescent conditions reckoned on the basis of the flammability limits evaluated at the mean temperature and pressure prevailing within the cylinder charge at the time of the spark. The measured limits in the engine were always higher than the corresponding flammability limit values for the three fuels. Both of these limits appear to correlate reasonably well with the calculated mean temperature of the mixture at the time of passing the spark.