We have determined the branching fraction for the production of CO from the reaction of methyl radicals and O(P-3) atoms at room temperature to be 0.18 +/- 0.04 at T = 296 K. The measurement was made by determining the concentration of CO using tunable infrared diode laser absorption. Methyl radicals and O atoms were prepared by 193-nm excimer laser photolysis of acetone and SO2. CO produced directly from acetone photolysis was distinguished from that generated by the radical-radical reaction by using acetone labeled with C-13 only on the methyl groups. The result is in excellent agreement with another determination of the CO branching fraction made using time-of-flight mass spectrometry.