The isobaric process of one-dimensional unsteady evaporation is considered for such a binary component system that liquid and gas phases are initially in different spacially uniform states. By virtue of the similarity nature of the flow fields involved, a set of ordinary differential equations for the heat flux and concentration as functions of temperature is derived, which is very useful in the examination of evaporation characteristics at high pressures. An asymptotic analysis is made of the heat and mass transfers which occur near the gas-liquid interface, just in a critical state of the binary mixture. It is especially shown that the binary mutual diffusion coefficient must vanish in such a critical state.