Measurements of the OH radical during winter show that noon-time concentrations at an urban site in Birmingham are only a factor of 2 less than during summer at the same site, despite a factor of 15 reduction in OH production from the photolysis of ozone. Noon-time concentrations of HO2 were similar for summer and winter. These high winter concentrations are quantitatively reproduced by a detailed box-model, showing that the winter radical pool (OH, HO2, RO2) is sustained by reactions of unsaturated hydrocarbons with ozone and by the photolysis of carbonyls. The high OH concentrations show there is significant chemical processing of volatile organic compounds (VOCs) in winter at mid-to-high latitudes in the northern hemisphere where emissions are high.