An equation is proposed on theoretical grounds for the description of isothermal equilibrium adsorption of vapors below their critical temperature on solid adsorbents. The proposed isotherm behaves as the Brunauer-Emmett-Teller (BET) equation in the monolayer region, the Anderson or Brunauer-Skalny-Bodor extension of the BET equation just above the monolayer, and the Frenkel-Halsey-Hill equation at higher coverage. Given suitable choices of its free parameters, the equation accounts for all isotherms from type I to type V of Brunauer classification.