The surface viscoelastic properties of monostearin and/or monoolein films spread on the air-aqueous phase interface were studied by sinusoidal oscillation tests performed in a special Langmuir trough with a cylindrical barrier. The surface rheological parameters - such as surface dilational modulus, elastic and viscous components, and loss angle tangent - and the surface tension were measured at 20 degrees C as a function of the time and radial frequency. The concentration of the lipid spread on the interface, the composition of the interface, and the aqueous phase composition (ethanol, 1 M; sucrose, 0.5 M) were the variables studied. It can be concluded that the films are essentially elastic. The surface dilational modulus is higher with monostearin than with monoolein but is not very frequency dependent. The surface rheological properties depend on the surface and subphase compositions. In the monostearin-monoolein mixed films the molecules are not miscible, so the surface dilational modulus is lower than in an ideal film.