The stability of liquid films adjacent to supersonic streams was investigated experimentally and compared with analytical results. Linear theories predict that films adjacent ta supersonic streams are much more unstable than those adjacent to subsonic streams. However, our supersonic experimental observa-tions indicated that the liquid film was stable with no entrainment under all test conditions. These conditions included laminar and turbulent boundary layers, a variation in the liquid Reynolds number (based on flow rate) from 1 to 200 and a variation in the free-stream unit Reynolds number from 1.6 × 106 m−1 to 110.0 × 106 m−1. These experimental observations can be explained by nonlinear theories, which predict that linear unstable disturbances do not grow indefhitely but achieve steady-state amplitudes in the supersonic case. The different aspects of the observed wave behaviour such as frequency, wavelength and amplitude are discussed and compared with previous experimental observations and the nonlinear theories.