The ratio of CO to CO2 produced during char combustion is an important parameter affecting the temperature of the char particle, the rate of mass transfer of gaseous species to and from the surface of the particle, and, consequently, the burnout time. In the present work, the ratio of CO to CO2 from combustion was measured for a biomass char using a thermogravimetric method. A sample of biomass char was combusted in conditions where the process was entirely limited by transfer of oxygen to the external surface of the fuel particles. The experimentally-obtained rate of combustion was compared with the expected rate of CO and CO2 transport, from which the ratio of CO to CO2 was calculated. The CO/CO2 ratio was determined for a char derived from birch wood and the results were compared with those from graphite and activated carbon at the same conditions. The experiments were conducted at various temperatures between 973 and 1173 K, with a range of partial pressures of oxygen in the oxidising gas from 0.0057 to 0.023 bar.The experimental CO/CO2 ratios were used to model the combustion of a single particle of char, predicting the total burnout time. Two models were developed: (1) an analytical solution for the burnout time assuming pseudo-steady state combustion, and (2) a numerical model of a char particle, accounting for gas transport and the combustion reaction inside the porous fuel particle. To validate the models, a separate set of experiments was performed where single particles of birch wood (6 mm dia.) were mounted on the tip of a thin thermocouple, then inserted into a hot reactor through which was flowing N2. The wood particle pyrolysed, leaving a single particle of char (∼4 mm dia.). The char was then burned in a laminar flow of oxidising gas. The burnout times obtained with both models agreed well with the experimental results. Commonly-used empirical expressions for CO/CO2 ratios from the literature were also employed in simulations but were found to under-predict the burnout time.