This work presents a new method of measuring the CO/CO2 ratio at the surface of carbon particles during combustion. This thermogravimetric method deduces the ratio of CO to CO2 by comparing the rate of consumption of carbon with the rate of oxidation of an external reference material with fast oxidation kinetics, in this case Cu. The method is useful when combustion is controlled by external mass transfer, commonly encountered in large-scale processes. The viability of this method has been demonstrated experimentally with graphite and a lignite char. It was found that in an atmosphere of ~ 1% O2, the graphite produced CO2 between 700 and 900 °C whilst the lignite char produced a mixture of CO and CO2 between 700 and 800 °C with the proportion of CO increasing with temperature, and above 850 °C, only CO was produced. It was also found that for this particular lignite char, the ratio of CO/CO2 increased with decreasing pO2 in the environment.