Thin-layer flat sorbents are used in analysis of alpha emitters allowing combine radionuclide separation and thin alpha source preparation into one stage. This work describes a comparative study of sorption of microamounts of Ra, Po, U, Th, Np and Pa by a thin-layer MnO2-CTA sorbent consisting of manganese dioxide coated onto cellulose triacetate. It was shown that pH dependences of Ra, U and Np sorption had S-type appearance with maximum in neutral and alkaline media that is typical for cation exchange mechanism of sorption. Sorption of Po and Pa was low over the whole pH range studied. Th showed unusual reverse S-type pH dependence of sorption with the maximum at pH 2.0–2.5 that can be explained by hydrolysis of thorium at pH > 4. Distribution coefficients of the isotopes were calculated using mathematical treatment of sorption isotherms. As it was shown on a real solid radioactive waste sample, the MnO2-CTA sorbent allowed separating Th-230 from Ra-226 and U-234, which emit alpha particles with similar energies. © 2023, Akadémiai Kiadó, Budapest, Hungary.