Field measurements of hydroxyl (OH) and hydroperoxy (HO2) radicals have been made in the remote Southern Ocean marine boundary layer at the Cape Grim Baseline Air Pollution Station in northwestern Tasmania. Measurements were made with a time resolution of 30 s over a period of 4 weeks during austral summer 1999 in the second Southern Ocean Photochemistry Experiment. Laser-induced fluorescence at reduced pressure was used to measure with an average detection limit at noon of (1.4 ¿ 0.5) ¿ 105 molecule cm-3 (1σ), for a 2.5 min integration period, and HO2 was measured following its conversion to OH via the addition of NO, with an average detection limit of (5.4 ¿ 1.7) ¿ 105 molecule cm-3 (1σ). Air masses arriving at Cape Grim originated from the Australian continent, Tasmania, and Southern Ocean/Antarctic regions, the latter (Baseline air) characterized by extremely low allowing a study of the chemistry of the unperturbed background atmosphere. In Baseline air, daytime average maxima for OH and HO2 of 3.5 ¿ 106 molecule cm-3 (0.14 parts per trillion by volume (pptv)) and 2 ¿ 108 molecule cm-3 (8 pptv) were observed, and a very high correlation coefficient (r = 0.95) was observed between the OH concentration and the rate of OH production from ozone photolysis. A steady state expression for OH, assuming production only from ozone photolysis and loss only by reaction with CO and CH4, overestimates the observed daytime maximum concentration in Baseline air by 20%. The HO2 concentration was found to depend upon the square root of the rate of O3 photolysis, as expected at very low concentrations of NO.