Pulsed-field-gradient NMR methods are used for determining the partitioning of surfactants between unimeric and micellar forms in mixed surfactant systems. The method allows determination of the composition dependence of activity coefficients in binary surfactant mixtures. Application of the regular solution approach to NMR data for several mixtures gives beta parameters which are dependent on concentration and which differ systematically among the surfactants within the same binary mixtures. The beta parameter summarizes interactions among different pairs of surfactants and, in principle, should not be composition dependent. The observed behavior is therefore not consistent with expectations from the regular solution model. Use of the van Laar expressions, on the other hand, accounts well for the composition dependence of the activity coefficients. The van Laar expressions also account for the often-observed composition dependence of regular solution beta parameters determined from CMC measurements. Though similar to the symmetric regular solution model, the van Laar expressions contain an additional parameter which reflects differences in the sizes of the mixture components. The results therefore suggest that headgroup size and headgroup packing are important contributors to nonideal surfactant behavior. Computer algorithms are described for extracting the van Laar interaction energy- and size-related parameters from NMR-derived results, from mixed critical micelle concentrations, or from heats of mixing. Data for several binary surfactant mixtures are presented and discussed. The results emphasize that when accurate data are available, the single-parameter regular solution model will not always fully account for nonideal surfactant mixing.