Phase behavior is a fundamental aspect of supercritical fluids. We describe a simple acoustic method for the investigation of fluid phase equilibria. The method is used to investigate pure components, and binary and ternary mixtures. In contrast to highly accurate speed of sound measurements in homogeneous fluids, we investigate the vapor-liquid boundaries of mixtures. The reciprocal of the speed of sound was measured at selected near-critical isotherms. For mixtures, isopleths are measured to establish the course of the critical line. All data for pure components are in good agreement with the literature where available. Limitations of the acoustic method in determining phase equilibria are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.