Peroxy radical measurements were made in the remote marine boundary layer over the Southern Ocean at Cape Grim, Tasmania during January and February 1995. From the experimental peroxy radical data it is clearly demonstrated that there is little night-time oxidation chemistry taking place. In such an environment at night the sole type of peroxy radical that persists is CH3O2 (&tgr;>12 hours) and the only nocturnal chemical process involving peroxy radicals appear to be the self-reaction of CH3O2 coupled to either a small but finite deposition velocity (kd~0.003 m s-1) or a slow reaction with O3 [k(CH3O2+O3) ≤8¿10-18 cm3 molecule-1 s-1>. A deviation between experimentally determined peroxy radical levels and J(O1D) in the late afternoon is discussed and modelled in terms of the peroxy radical self-reactions. ¿ American Geophysical Union 1996