Methyl radicals react with macrocyclic cobalt complexes (H2O)2LCo2+ (L1 = 1,4,8,11-tetraazacyclotetradecane, L2 = C-meso-5,1,1,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) in the presence of CO in acidic aqueous solution. The reaction yields a mixture of methyl and acetyl complexes as a result of the competition between CO and (H2O)2CoL2+ for CH3 Kinetic competition experiments yielded the rate constant for the capture of CH3 by CO, kCO = (2.0 ± 0.3) X 106 M-1 s-1. The acetyl complex of CoL1 crystallizes with either H2O or ClO4- coordinated trans to the acetyl group. Both the red [(ClO4)L1CoC(O)CH3](ClO4) (1) and the yellow [(H2O)L1-CoC(O)CH3](ClO4)2H2O (2) crystallize in the centric space group P21/c with four molecules in a unit cell of dimensions a = 10.539 (1) Å, b = 12.548 (2) Å, c= 14.915 (2) Å, and β = 93.74 (3)° for 1 and a = 12.588 (3) Å, b = 8.821 (3) Å, c = 19.398 (5) Å, and 0 = 96.34 (2)° for 2. The data refined to final values of the unweighted R factors of 0.037 (1) and 0.054 (2).