An apparatus for high pressure studies of phase equilibrium in multicomponent fluid systems is completely described. The apparatus has been designed to yield a complete thermodynamic description of fluid mixtures containing any number of components which have up to three coexisting phases in the pressure range up to 100 MPa and for temperatures up to 423 K. It therefore makes possible the determination of liquid and vapour phase densities and composition at prescribed pressures and temperatures for two phases as well as the determination of the same quantities for a second liquid phase should one exist. At present, owing to the temperature and pressure limitations imposed by some of the commercial parts, the apparatus works within the temperature range up to 393 K and the pressure range up to 40 MPa. The instrument has the capability for complete automation if it is desired. The results of preliminary measurements in the normal hydrocarbon systems CH4-C6H14 and CH4-C6H14-C14H30 are presented.