Investigations were conducted on the range of emission rates of NO, NO2, CO and hydrocarbons, and the thermal efficiencies produced by each cooktop burner on three production cookers, supplied by different Australian manufacturers. Factorial experimental design was used to provide a systematic and definitive approach to the investigation. It was found that the emissions and efficiencies were markedly affected by (1) the thermal input, both with and without a load on the burner, and (2) the load-height to flame-length ratio, with a load on the burner. The effect of hydrocarbon leakage into the secondary/dilution air was investigated, to explain the observed correlation of hydrocarbon emission rate with dilution air flow. The hydrocarbons were found to arise from two sources: leakage from the stabilisation ports at the base of the burner, which was found to be significant at the higher thermal input; incomplete combustion of the fuel in the premix, which was the dominant source of hydrocarbons emission at the lower thermal input. The presence of hydrocarbons in the secondary/dilution air was found to raise the ratio of NO2/NOx slightly from 0.23 to 0.27 at the higher thermal input, and to increase the measured emission rate of carbon monoxide, but to have no effect on the total NOx emission rate. At the lower thermal input, hydrocarbons derived from flame-quenching or incomplete combustion on the burner enhanced the conversion of NO to NO2, giving NO2/NOx ratios up to 0.7.