The results of studies on the interaction of binary mixtures of sodium deoxycholate (NaDC) and polyoxyethylene sorbitan monooleate (Tween 80) in bulk and at the air/water interface obtained from conductance, surface tension, and fluorescence measurements are described. The critical micelle concentration (CMC), thermodynamics of micellization, free energy of interfacial adsorption, minimum average area occupied by the surfactant species at the interface, micellar polarity, and aggregation number of the mixed aggregates have been determined. The mixed micellar composition and the estimation of the interacting forces involved are evaluated on the basis of recent theoretical models. The estimated interaction parameter indicates an overall attractive force in the mixed state, and the proportion of NaDC in the mixed micelle is found to be lower compared to the stoichiometric compositions. The mixed aggregates with higher mole fractions of NaDC show less stability in comparison with those having higher proportions of the nonionic component,