Seven mixtures of formaldehyde and oxygen diluted in argon were studied behind reflected shock waves at temperatures from 1340 to 2270 K and pressures from 0.7 to 2.5 atm. Mixture compositions covered a range from pure pyrolysis to lean oxidation at a stoichiometric ratio of 0.17. The progress of reaction was monitored by laser absorption of CO molecules. Experimental rates of CO formation were found to be 80% higher, in the case of pyrolysis, and 30% lower, under lean oxidation, than those predicted by the current reaction model, GRI-Mech 1.2. The collected experimental data were subjected to extensive detailed chemical kinetics analysis, including optimization with the solution mapping technique. The analysis identified a strong correlation between two rate constants. Assuming a recent literature expression for one of them produced k(-1a) = 2.66 x 10(24)T(-2.57)e(-215/T) cm(6) mol(-2) s(-1) for the reaction H + HCO + M --> CH2O + M. A new expression was developed for the reaction HO2 + CH2O --> HCO + H2O2, k(6) = 4.11 x 10(4)T(2.5)e(-5136/T) cm(3) mol(-1) s(-1), by fitting the present and literature results. With these modifications, the new reaction model provides good agreement with our experimental data and an acceptable agreement with most literature experimental observations.