The critical layer for thermal escape of a planetary atmosphere is treated on a model in which an isothermal exosphere at high temperature merges within a narrow zone (the endopause) of varying temperature into the remainder (the endosphere) of the atmosphere, isothermal above the level at which mixing stops. Only one major constituent of the atmosphere is considered, and constant scale heights are assumed for the exosphere and for the endosphere above the level at which diffusion sets in. The height of the escape layer is evaluated analytically by considering the pressure balance at the base of the exosphere. The result permits one to determine explicitly the amount (the concentration or the number of molecules above unit area) of a minor constituent at the critical level, taking thermal diffusion across the endopause into account. The predicted height of the escape layer as determined from a model atmosphere of Nicolet shows essential agreement with the height inferred directly by means of the number density, scale height, and collision diameter.