The calculation of phase diagrams for binary and multicomponent Group III-V semiconductor systems has drawn a great deal of attention recently. A simple formalism for the calculation of such diagrams which includes a consistent representation of standard state properties is presented. The ability of a number of solution models to represent the liquidus and thermochemical data for the In-Sb and Ga-Sb systems is evaluated. It is found that the non-random two-liquid theory (NRTL) best represents the asymmetric and temperature-dependent properties of the In-Sb system, but little advantage is gained in the use of the NRTL equation over the simple solution model for the Ga-Sb system. The problems encountered when calculating thermochemical properties from liquidus fits and performing analogous procedures are also considered.