The nonideal solution treatment of molecular interactions in binary mixtures of surfactants at a solid-liquid interface elaborated by Rosen et al. has been used to study the synergetic phenomena that can take place at the cholesterol-bile salt solution interface. The three binary combinations of sodium cholate (NaC), sodium deoxycholate (NaDC), and sodium chenodeoxycholate (NaCDC) in aqueous solution were investigated. Synergism was found in NaC-NaDC and NaC-NaCDC mixtures, and no synergetic effect was detected for the NaDC-NaCDC mixture. For all the systems studied there was consistency between synergism found at the solution-air and at cholesterol-solution interfaces. Values of the molecular interaction parameter, beta, at the solution-air and at the cholesterol-solution interface were also calculated. The small values of beta in the three binary systems, especially in the NaDC-NaCDC mixture, reflect a weak interaction between the surfactant molecules at the interface. This was ascribed to the similar molecular structures of these surfactants.