Explicit expressions for local surfaces and local compositions in mixtures obtained from the quasi-chemical approach of Guggenheim are presented and their relations with empirical expressions discussed. The treatment is greatly simplified by introducing well-defined, non-random factors. A method is proposed for the solution of the set of non-linear equations arising from the quasi-chemical approach. Detailed equations are given for the case of binary and ternary systems. The extension of the general approach in terms of groups is discussed. The quasi-chemical expressions for the local surface fractions are combined with the formalism of the New Flory Theory in order to account for non-randomness in polymer solutions. The resulting expressions are applied to experimental data of the systems carbon tetrachloride—polypropylene oxide, chloroform—polypropylene oxide and benzene—polyisobutylene.