The kinetics of carbon monoxide disappearance in the afterflame zone of ethylene/oxygen and ethane/oxygen flames were investigated. The composition profiles of stable species such as CO, CO2, O2, H2O, N2 and Ar were determined by subjecting the gas samples (taken with a triple walled water-cooled stainless steel probe) to mass spectrometric analysis. The profiles of hydroxyl radical, an unstable species, and temperature were obtained by using ultraviolet absorption spectroscopy. Theoretical multicomponent diffusion coefficients were used for calculating the net reaction rates from the composition and temperature profiles. The overall reaction rates for carbon monoxide and carbon dioxide were also calculated from a postulated reaction mechanism for the CO/O2/H2 system. Theoretical flame structures were calculated by numerical integration of the coupled kinetic rate equations and the diffusion equations. (Modified author abstract)