A mechanism for the atmospheric oxidation of hydroxymethyl hydroperoxide (HMHP) by hydroxyl radicals (OH) is proposed from an ab initio study. Using CCSD(T)/6-311++G(2df,2p)//MP2/6-31G(d) level of theory, the Structures and energetics of intermediates and transition states involved in the atmospheric oxidation of HMHP are examined. Critical structures also are studied by multireference techniques (CASPT2 and MR-SDCI) to ensure the reliability of the results. From these calculations, reaction enthalpies and barrier heights are presented for three different reaction pathways for the atmospheric oxidation of HMHP. The oxidation reaction can be initiated by hydrogen abstraction at three different positions. Results suggest that the hydrogen abstraction from the carbon is the dominant pathway because this is the process with the lowest entrance barrier. This H abstraction is followed by spontaneous decomposition without any stable intermediates and leads straight to the formation of formic acid and OH radicals. Thus, an oxidative degradation of HMHP into HC(O)OH and water Occurs that is catalyzed by OH radicals. These results support the suggestion that HMHP Could be an atmospheric source of formic acid. The two competing but less favorable oxidation pathways may contribute to the HOx abundance in the atmosphere. A new minor byproduct of the oxidation of HMHP predicted from the calculations is HC(O)OOH.