The atmospheric mixing ratio of formaldehyde (HCHO) was measured continuously in clean marine air at the Australian Baseline Station at Cape Grim between mid November and mid December 1993. A diurnal cycle in mixing ratio was observed, consistent in amplitude with the expected photochemical source of this species. However the absolute values of the HCHO mixing ratio were higher than expected if the major source of HCHO under clean, low-NOx conditions is photolysis of methyl hydroperoxide (CH3OOH) derived from oxidation of methane by OH radicals. Possible explanations for elevated HCHO levels are considered. One sufficient to explain the observed HCHO levels is that reaction between hydroperoxy (HO2) and methylhydroperoxy (CH3O2) radicals may not proceed with 100% efficiency to form CH3OOH, but may have an additional branch yielding HCHO in clean marine conditions. There is some evidence from laboratory studies consistent with this proposal.¿ 1997 American Geophysical Union