A theoretical study of the mechanism and kinetics of the CH(X-2 Pi) + H2C=O reaction was carried out by ab initio molecular orbital theory based on the CCSD(T)/aug-cc-pVTZ//BHandHLYP/aug-cc-pVDZ method in conjunction with statistical theoretical kinetic VTST and RRKM Master Equation calculations. The potential energy surface for the cis/trans-HCOH + CH reactions was also examined. Calculated results show that the association reaction of CH and CH2O occurs by addition of the CH radical onto the oxygen atom, cycloaddition onto the C=O bond, and, for a small fraction, insertion of CH into a C-H bond, forming CH2C-O-CH, cyclic H2COCH, and CH2CHO, respectively. These channels are all banierless, leading to a rate coefficient near the collision limit with a slight negative temperature dependence, in excellent agreement with experimental data. The intermediates can undergo extensive isomerization across seven C2H3O isomers, many with multiple conformers, prior to fragmentation. Eight fragmentation product sets were characterized, where H2CCO + H and CH3 + CO were found to be the major products at lower temperatures, while (CH2)-C-3 + HCO started to contribute at higher temperatures. CCHO + H-2, C2H + H2O, HCCOH + H, C2H2 + OH, and HCCO + H-2 have negligible contributions for temperatures below 3000 K and pressures up to 100 atm. Collisional stabilization of the C2H3O isomers is negligible except at the highest of pressures and low temperatures.