Using numerical methods, the authors investigate plasma diffusion in tokamaks when impurity ions of light elements are present. The model considered makes use of neoclassical expressions for diffusion fluxes and takes ionization and recombination processes into account. The question of the boundary conditions for the initial system of equations is discussed. The new boundary conditions for the proton and impurity ion densities are zero partial ion-ion fluxes at the boundary of the plasma column. The computational results show that the presence of impurity ions in the plasma column of a tokamak – even in relatively small amounts – strongly influences the spatial distribution of the protons. The entire diffusion process can be broken down into a number of stages which differ one from the other with regard to the amount of impurities in the system. If the proportion of impurities ξim does not exceed 0.01 of the electron density, the highly ionized ions collect at the centre of the plasma. If ξim > 0.01, the proton profile becomes flat within a short time and the impurities become more evenly distributed over the plasma cross-section.