We analyze adsorption of a multicomponent mixture at high pressures on the basis of the potential theory of adsorption. The adsorbate is considered as a segregated mixture in the external field produced by a solid adsorbent. We derive an analytical equation for the thickness of a multicomponent film close to a dew point. This equation (asymptotic adsorption equation, AAE) is a first order approximation with regard to the distance from a phase envelope. It can be applied to both normal and retrograde condensation. Different forms of the AAE make it possible to analyse phenomena occurring close to a dew point. For testing of the AAE, we compare thicknesses of the adsorbed film predicted by this equation and obtained by direct calculations.