An analytical apparatus with improved recirculating, and sampling mechanisms was presented to investigate phase behavior of binary and multicomponent systems, which effectively avoided undesirable pressure gradients across the cell and serious disturbance of equilibrium during sampling procedure. Two one-channel magnetic recirculation pumps were designed and built to provide better mass transfer between the coexisting phases. Two six-port switching valves were used for sampling, quantitative fluid injection, and online compositional analysis by gas chromatography. The apparatus was validated by means of isothermal vapor-liquid equilibrium data for CO2 + hexane, arid the results were found to be in good agreement with the literature data. High pressure phase equilibria data of CO2 + n-alkanes and CO2 + crude oil + n-alkanes were measured from 353.15 to 373.15 K. Three thermodynamic models of RR, PRSV, and YQE, based on Peng-Robinson and a modified quartic equation of state coupled with various mixing rules, were suggested to represent phase equilibrium. Calculations were compared with experimental values for evaluating the predictive ability of these proposed equations. Analysis of the results confirmed that the YQE model was superior to all the other equations examined and worked well in describing phase behavior of complex real fluids.