The theoretical analysis about the influence of high concentration and large concentration gradients on the hydrodynamics and mass transfer in the approximations of the laminar boundary layer has been done. The results obtained show that the change in the density with the concentration influences the hydrodynamics in gases and liquids and does not influence the mass transfer in gases. The change in the viscosity with the concentration influences the hydrodynamics in gases and liquids and the mass transfer. The change in the diffusivity with the concentration does not influence the hydrodynamics and the mass transfer. It has been clearly seen that the non-linear theory on mass transfer at the constant values of density, viscosity and diffusivity has enough accuracy for gases and liquids, if the density of transferred substance is not sufficiently different from the density of the gas mixture.