The article describes the results of experiments on sorption of various alpha emitting radionuclides on thin-layer sorbents consisting of manganese dioxide layer coated onto polymer films. In these experiments, we have faced a phenomenon of increase of full width at half maximum of peaks at alpha spectra of the sorbents with the increase of sorption time. We have explained this phenomenon by diffusion of alpha emitters into the sorbents. We have suggested a method of mathematical treatment of the alpha spectra that allowed calculation of diffusion coefficients of the alpha emitters in flat samples. This method is suitable for cases of diffusion of radionuclides (T½ = several days to 106–107 years) in flat samples if time of diffusion is enough for migration of a radionuclide into the depth of the sample by at least 0.1–0.5 µm. The method was tested for the thin-layer manganese dioxide sorbents after sorption of Ra-223, U-233, Np-237, Th-230 and Po-210. It was found that mobility of the studied radionuclides in manganese dioxide layer followed in a series NpO2+ > Np4+ > Ra2+ ≈ UO22+ ≈ Th4+ > Po2+. The values of diffusion coefficients varied from 1.3 × 10–20 m2/s for Po-210 to 5.8 × 10–17 m2/s for Np-237.