A method is proposed for the measurement of self-diffusion coefficients in condensed media. To this end, a substance (or a mixture of substances) is brought into contact with the same substance (mixture), differing by the isotope composition, directly in the ampule of an NMR spectrometer or NMR relaxometer. Then, the integral intensity of the NMR signal, or the maximum amplitude of the signal of free precession, is measured in both parts of the ampule, and the diffusion-induced time variations of the signal are monitored. Using this method, the self-diffusion coefficients were determined in an H₂O-D₂O mixture, in a mixture of light and heavy sulfolane in the plastic phase, and in a polyurethane-dioxane system. The self-diffusion coefficients measured by this technique agree well with the values obtained by other methods.