A theoretical analysis of the non-stationary evaporation kinetics based on experimental data has been done. A comparison between non-stationary diffusion rate and convective mass flow as a result of a Stephan flow is proposed. It is shown, that the evaporation do not change the temperature of the liquid interface. The mass transfer rate as a result of a diffusion and convection differs from the evaporation velocity, which is-easy to explain with a natural convection on account of a instability of the system. (C) 2003 Elsevier Science Ltd. All rights reserved.