Phase behavior is a fundamental aspect of supercritical fluids. A simple acoustic method is described for investigating vapor/liquid equilibria. The method is used lu investigate three different pure components (CO2, C2H6, and CF3CH2F, refrigerant R134a) and binary mixtures of CF3CH2F with CO2 or C2H6. For the pure components, the reciprocal of the speed of sound was measured at selected temperatures around the critical point. The data obtained are in good agreement with literature values. The binary mixtures were investigated over the whole composition range (i.e., mole fraction of CF3CH2F from 0 to 1.0), and their critical curves were determined. The critical lines show the expected type I fluid phase behavior, but the critical Line in C2H6 + CF3CH2F shows an unusual pressure minimum and maximum. The resulting critical Lines are discussed with respect to thermodynamics, the use of these mixtures for supercritical fluid chromatography and extractions, and the possibility of using CF3CH2F as a modifier.