Emissions measurements made during type-testing of domestic gas appliances are subject to some uncertainty, which derives partly from non-reproducibility of the emission itself. One cause is variations in composition of the gas. The CEN Standards for test gases set Wobbe Number limits of +/-2%, but do not control the non-methane content effectively. Even at constant Wobbe Number, however, composition can affect emissions. New tests show that, for methane-based natural,gases at constant Wobbe Number, there is a relationship between NOx emission and propane content. This is modified for the inclusion of ethane or inert gas, which are the other main non-methane components. An empirical formula is proposed for correcting the measured NOx emission to its equivalent for pure methane, allowing also for the limited Wobbe Number effect. For CO, there is a rising-then-falling relationship with non-methane content, but no general correction formula was possible.