Using kinetic theory approach, the fluid- and thermodynamics aspects of a vapor next to its interphase surface is studied under conditions of arbitrarily strong interphase processes in single component systems. The physical domain considered is a boundary layer known as the Knudsen layer, a few molecular mean free paths thick, in which the vapor adapts to given external equilibrium conditions. It is shown that the limiting flow conditions in strong evaporation closely corresponds to the sonic state. Expressions for the mass transfer are derived both in the general and in the linear cases, and comparisons are made with the classical Hertz-Knudsen and Schrage formulae which are both shown to be incomplete. The vapor is found to be far from its saturated state, both at the interphase surface and at the border of the continuum region. The coupling between the Knudsen layer and this latter region is shown by an example of evaporation into a shear flow.