Experimental area/molecule data (A(expt)) at the air/aqueous solution interface after mixing, ideal mixing area/molecule data (A(ideal)), based upon area/molecule data at that interface before mixing, and regular solution theory have been used to explain the values of surfactant molecular interaction (beta) parameters observed in mixed monolayers and mixed micelles. The value of the beta parameter reflects the difference in surfactant-surfactant interactions before and after mixing. In ionic-nonionic surfactant mixtures, when surfactant-surfactant interactions are weak, reduction in electrostatic self-repulsion interaction energy of the ionic surfactant, due to the dilution effect upon mixing, is suggested as a major contributor to the negative beta values observed for mixed monolayers and mixed micelles. Steric effects appear when surfactant molecular structure varies in the size of the headgroups and in the branching of the hydrophobic groups. It is suggested that the effect of an increase in the size of the hydrophilic group of the ionic surfactant in anionic-nonionic surfactant mixtures is to decrease electrostatic self-repulsion of the ionic surfactant before mixing, producing less negative beta values. In contrast, the observation of more negative beta values with increased length of the polyoxyethylene chain in POE nonionic-anionic surfactant mixtures, which increases the size of the hydrophilic group of the nonionic surfactant, may be due to the acquisition of a positive charge by the polyoxyethylene group in the nonionic surfactant upon being mixed with an anionic surfactant, which would cause attractive electrostatic interaction with the anionic surfactant in both the mixed monolayer and mixed micelle after mixing. Bulkiness due to branching in the hydrophobic group of the ionic surfactant decreases electrostatic self-repulsion before mixing with a nonionic surfactant having a linear hydrophobic group. Branching in the hydrophobic group of a polyoxyethylenated nonionic surfactant causes some contraction upon mixing with an anionic surfactant with a linear hydrophobic group, due to the greater steric repulsion in the bulky molecule of the nonionic surfactant before mixing and its reduction after mixing, resulting in slightly more negative beta(sigma) values than with the corresponding linear hydrophobic chain nonionic surfactant.