A new modification of matrix-isolation EPR has been developed for the measurement of uptake coefficients of polyatomic radicals. The technique is combined either with a cylinder reactor having a movable source of radicals or a coaxial reactor having a movable rod and a fixed source of radicals. The walls of the cylinder reactor or the rod surface are covered with the aerosol material. A simple way to treat the experimental data on the basis of the kinetic resistance additivity formula is proved. The results of HO₂ and CH₃O₂ uptake coefficient measurements are given and the possible implications for atmospheric chemistry are discussed. The measured value of the HO₂ diffusion coefficient was closer to D H₂O₂ than to D O₂ . The HO₂ uptake coefficient on the concentrated 80 and 96% w/w H₂SO₄ was >0.2 at 243 K. For NaCl, KCl, NH₄NO₃ and (NH₄)₂SO₄, the γ HO₂ values were in the range (1–2)× 10^–2 and γ CH₃O₂ = 0.3 × 10^–2 at 295 K for NaCl. No changes in γ HO₂ were found at [HO₂]= 4 × 10⁹–3 × 10¹¹ cm^–3. The results are discussed from the viewpoint of stratospheric ozone in volcanic clouds and tropospheric hydroxyl abundance in urban regions.