The classical solution of the Stefan diffusion is based on the "plausible intuitive hypothesis" (Wilke. 1950) which assumes the stagnancy of the inert component (N-B = 0). The considerations performed in this Study as well as the review of the main publications concerning the Stefan diffusion lead to an explicit conclusion that the principal phenomena underlying the Stefan diffusion are the diffusion of components and the viscous bulk flow coupled with each other. In order to determine the accuracy and the range of application of the classical solution (N-B = 0) relationships have been derived estimating the intensity of the entropy sources of the diffusion process with a stagnant inert component as well as of the viscous bulk flow (creeping flow), generated by the diffusion. Assuming that the rate of the entropy production is the measure of the intensity of each partial mechanism, the ratio of these quantities defines their participation in the Stefan diffusion process. As a consequence it will establish a criterion defining the region of application of Eq. (4).