In order to describe quantitatively the nature and composition of the surface layer in a binary liquid mixture, some equations are derived within the frame of the association solution model by Kretschmer and Wiebe. Bulk concentrations, properties of the pure compounds (surface tension sigma, molar volume v and association constant K) and the constant for complexation K-AB are involved in such expressions. If molecules of both compounds are not able to form heterocomplexes only one adjustable parameter is necessary, while another one is included when K-AB not equal 0. The proposed set of equations acquires special significance when heteroassociation compels the surface tension of a solution to be higher (or lower) than that of both pure compounds. Different binary systems have been employed to test the obtained relationships, namely (THF + n-hexane, n-heptane, n-octane, n-nonane or n-decane), (ethanol or 1-propanol + n-hexane or n-heptane), and (THF + 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol or 1-decanol). Surface tension values for these mixtures were taken from the literature except those for the series (THF + 1-alkanol), which have been measured at the temperature 298.15 K by using a drop volume tensiometer. A simulated annealing method has been utilized in order to carry out the adjustments. Surface tension data are well described by the proposed set of equations in all cases.