The heterogeneous interactions of OH and HO2 radicals with a number of surfaces characteristic of atmospheric particulate matter have been studied by using a low-temperature flow tube coupled to a resonance fluorescence detector. In particular, the mass accommodation coefficients (a) of both OH and HO2 on supercooled sulfuric acid solutions have been measured : for OH, alpha > 0.2 for 45-96 wt % solutions from 220 to 295 K, and for HO2, alpha > 0.2 on 55 wt % solutions doped with 0.1 M CuSO4 at 223 K. Radical uptake coefficients (gamma) were also measured on a variety of solid surfaces prevalent in the atmosphere : water ice, NH4HSO4 and (NH4)(2)-SO4. On water ice, although it was found that the uptake coefficients of both OH and HO2 were relatively small on conditioned surfaces (gamma(OH) = 0.03 +/- 0.02 from 205 to 230 K, gamma(HO2) = 0.025 +/- 0.005 at 223 K), it was observed that the OH uptake coefficient could be significantly increased by either adsorbing HNO3 to the surface or melting the ice surface by exposure to relatively high partial pressures of HCl. Similarly, on conditioned (NH4)(2)SO4 and NH4HSO4 surfaces at room temperature, the OH uptake coefficient is relatively small, gamma(OH) < 0.03. Nevertheless, the uptake coefficient can be significantly increased (gamma(OH) > 0.2) by exposing the surface to relatively low partial pressures of an organic species, 1-hexanol.