Abstract—From flash photolyses of methyl formate (HCOOCH3), d‐methyl formate (DCOOCH3), methyl formate‐d3 (HCOOCD3) and fully ***deuterated methyl formate (DCOOCD3) in the vapour phase at room temperature, the relative efficiencies of the formyl and methoxyl radicals in producing formaldehyde have been determined.