Fisher model of intercrystalline diffusion is refined based on an analysis of previously obtained results of emission Mössbauer studies of grain boundaries. It is shown that under conditions of dominating intercrystalline diffusion, the existence of monatomic near-boundary layers of equilibrium composition should be taken into account. It has been found that the allowance for these layers does not change the expressions for the distribution of the concentration of the diffusing impurity in a polycrystal in the case of a type-B kinetic regime, whereas in the type-C regime the corresponding expressions change somewhat. The technique that was used previously for the determination of parameters of grain-boundary diffusion and segregation, which is based on the results of emission Mössbauer investigations of grain boundaries, has been modified; the application of the new approach has been demonstrated on the example of processing data of Mössbauer investigations of grain boundaries in polycrystalline Nb at ^119mSn nuclei.