An effective and inexpensive method is described for measuring critical parameters by acoustic and photoacoustic techniques. The apparatus is based on a miniature cell with windows, transparent to both UV and IR radiation, and two piezoelectric transducers. Its use is illustrated by measurement of the critical parameters of pure C2H6 and CO2 and of mixtures of these gases with N-2 and H-2. The technique has considerable potential for establishing the phase behavior and composition of supercritical mixtures.