Recently Tanford and Pease have shown that in a number of carbon monoxide flames the burning velocity increases as the calculated equilibrium concentration of hydrogen atoms in the flame front increases. They point out that this would be so if the propagation of the flame were dependent on the diffusion of hydrogen atoms ahead of the flame. It is shown in the present paper that, by combining calculated ignition and flame temperatures, the variation in burning velocity with composition can also be accounted for on an essentially thermal theory of flame propagation.