Mixing ratios of individual organic hydroperoxides were measured during summer 1990 on a mountain summit (1780 m a.s.l.). The dominating compound during the early summer months was hydroxymethyl hydroperoxide (HMHP) while later in the summer methyl hydroperoxide (MHP) prevailed. HMHP and MHP together made up more than 90% of total organic hydroperoxides (ROOH) which constituted about 10--40% of hydrogen peroxide measured concurrently at the same site. The ROOH mixing ratios at an altitude of 1175 m a.s.l., representing the planetary boundary layer (PBL), were about twice as large as those at the summit. Higher ROOH values in the PBL point to ROOH formation from anthropogenic and biogenic hydrocarbon precursors. The tendency to higher HMHP values in early summer points to the possible HMHP formation from biogenically emitted unsaturated hydrocarbon precursors.