The assumption of a unimolecular layer as the surface phase separating a strictly regular solution from its vapour phase, is in contradiction with the Gibbs adsorption formula. When, however, one takes into account the possible presence of two unimolecular layers inside the surface phase, it is shown that the composition of the layer on the side of the solution differs but little from the composition of the solution itself, a circumstance which justifies the unimolecular layer model as a first approximation. The surface tension is not appreciably altered by this treatment, but the relative adsorption is modified significantly. The formulae thus obtained are no longer in contradiction with the Gibbs formula.SCOPUS: ar.jNOTXTinfo:eu-repo/semantics/publishe