An expression for calculating the molar concentration ratio of CO/CO2 produced on a burning carbon surface is presented. Three reactions on the surface of the carbon particle burning in air are considered: (a) C + O-2 --> CO2, (b)C + 1/2O(2) --> CO, and (c)C + CO2 --> 2CO; whereas in the gas phase surrounding the particle there is the homogeneous reaction: CO + 1/2O(2) --> CO2. The molar concentration ratio of primary product CO to CO2 formed by reactions a and b on the carbon surface is CO/CO2 = g(co(b)/g(co2(a)). M(co2)/M(co), where g(co(b)) is the mass flux of CO by reaction b on the carbon surface, g(co2(a)) is the mass flux of CO2 by reaction a on the carbon surface, whereas M(co) and M(co2) are the molar weights of CO and CO2, respectively. To find this ratio, the burning rate and temperature are measured. Based on experiments and the solution of a set of equations for a large carbon particle (3-mm diameter), the oxygen content on the surface of the burning particle is calculated. This gives the reaction order and frequency factor of the reactions C + 1/2O(2) --> CO and C + O-2 --> O-2. The surface ratio of CO/CO2 can be obtained, provided the surface oxygen concentration is known. The advantage of this method is that the burning rate and temperature can be accurately measured. Good agreement between experiment and calculation shows the proposed expression for CO/CO2 to be an improvement on previous ones.