OH and HO<sub>2</sub> radical concentrations have been measured in the boundary layer of coastal Antarctica for a six-week period during the austral summer of 2005. The measurements were performed at the British Antarctic Survey's Halley Research Station (75° 35' S, 26° 19' W), using the technique of on-resonance laser-induced fluorescence to detect OH, with HO<sub>2</sub> measured following chemical conversion through addition of NO. The mean radical levels were 3.9×10<sup>5</sup> molecule cm<sup>−3</sup> for OH, and 0.76 ppt for HO<sub>2</sub> (ppt denotes parts per trillion, by volume). Typical maximum (local noontime) levels were 7.9×10<sup>5</sup> molecule cm<sup>−3</sup> and 1.50 ppt for OH and HO<sub>2</sub> respectively. The main sources of HO<sub>x</sub> were photolysis of O<sub>3</sub> and HCHO, with potentially important but uncertain contributions from HONO and higher aldehydes. Of the measured OH sinks, reaction with CO and CH<sub>4</sub> dominated, however comparison of the observed OH concentrations with those calculated via the steady state approximation indicated that additional co-reactants were likely to have been present. Elevated levels of NO<sub>x</sub> resulting from snowpack photochemistry contributed to HO<sub>x</sub> cycling and enhanced levels of OH, however the halogen oxides IO and BrO dominated the CH<sub>3</sub>O<sub>2</sub> – HO<sub>2</sub> – OH conversion in this environment, with associated ozone destruction.