This research develops a theoretically based corresponding states procedure which predicts mixture properties accurately. This procedure separates any dimensionless residual thermodynamic property into two parts: One part is a contribution from molecular repulsion and the other is a contribution from molecular attraction. This repulsion can be represented by either a mixture of hard spheres or hard convex bodies. The two forms are compared in this work. The attraction contributions contain the symmetrical part from non-polar interactions and another asymmetrical part which comes from permanent dipole moments, quadrapole moments, or other polar effects. Each of these two attraction contributions in a mixture is determined from a pure reference fluid by way of some suitably defined pseudoparameters. The composition dependence of the pseudoparameters is derived from either the hard sphere expansion of the hard convex body expansion method. Temperature and density dependent shape factors are used to multiply the individual critical properties in the pseudocriticals to establish conformality between individual constituents and the reference. A new analytical method of defining optimal values for the hard core dimensions is developed for each pure component from its equation of state. This new conformal solution theory gives good agreement with experiment and always gives better results, especially for K-value calculations, than the results determined from the ordinary equation of state with the usual empirical mixing rules. This work has confirmed that this new theory gives a better description of composition dependence than the empirical combinational rules. The hard convex body expansion method does not give significant improvement over the hard sphere expansion results for a methane-propane mixture at the conditions investigated in this work. A reliable equation of state which describes the behavior of the pure component accurately is essential to the computation. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UMI