The isothermal oxidation of C-a formed by CH4 dissociation has been studied on Pt{110} using molecular beam techniques at a surface temperature of 650 K. The reaction products CO and COY are produced in the ratio of 9:1, respectively, for all carbon coverages studied (0.03-0.8 ML). When the O-2 beam is turned on, there is a sharp rise in the rate of CO production (less than 0.1 s) which is attributed fur the first time to the surface reaction of C-a with short-lived chemisorbed molecular dioxygen, O-2c. This is followed by a slow rise to a maximum rate that is due to the surface reaction between C-a and adsorbed atomic oxygen, O-a, which accumulates more slowly on the surface. The initial rate of CO formation from O-2c decreases as the C-a, precoverage is increased, while the time taken to reach the maximum in the rate front the C-a + O-a reaction increases. The CO rate maximum always occurs earlier than that of CO2, for all carbon coverages. The rate constant for the C-a + O-2c reaction decreases by a factor of 6 up to a C-a precoverage of 0.2 ML and then stays almost constant up to saturation.