The surface dilational properties (i.e. surface dilational modulus, elastic and viscous components, and loss angle tangent) and surface tension of mixed emulsifiers (bovine serum albumin (BSA), monostearin, and monoolein) were determined using a modified Langmuir trough. The surface dynamic properties were measured at 20 degrees C, as a function of time, interfacial composition (BSA-monostearin and BSA-monoolein mixtures), subphase composition (aqueous solutions of ethanol and sucrose), and compression-expansion frequency. The films displayed a viscoelastic behavior and were frequency-dependent. The surface dilational modulus increased as the concentration of monostearin spread on the interface increased, but it decreased as the concentration of monoolein spread on the interface increased. In the presence of ethanol or sucrose in the interface at 1.0 M, the surface rheological parameters decreased. These phenomena have been related to protein-lipid interactions at the interface as a function of the aqueous phase composition.