The inhibiting action of HCHO on the first and second limits of the H2+02 reaction has been studied in KClcoated vessels at 540°C. The e5ciency of inhibition is almost inversely proportional to the mole fraction of 0 2 , and is less dependent on the mole fraction of H2. This indicates that the main primary termination process is thereaction H+HCH0( 14) which competes with the reaction H+02(2); a smaller contribution results from the reaction of OH+HCHO(lS) competing with OH+H2(1), or from O+HCHO = OHSHCO(13) competing with O+H2 = OH+H(3). The efficiency of inhibition is independent of diameter showing that the HCO radicals are not destroyed at the surface ; this is confirmed by the fact that the fraction of HCO radicals undergoing chaintermination reactions is effectively independent of mixture composition and of pressure over the range 4-100 mrr~ Hg. Evidence against the reactions HCO = H+CO and HC0+02 = C02+0H is provided, and it is concluded that chain termination ultimately results either from the reaction HC0+02 = CO+HO2, or from the reaction HCO+02 = HCO3, the HCO3 radicals being destroyed at the surface. The results permit the evaluation of the ratios k14/k2 and kls/kl(or k13/k3) and from estimates of k14, a value of k2 can be obtained which agrees with earlier estimates.