We consider the phase diagrams of hard sphere mixtures from the point of view of some well known approximations in classical thermodynamics which have been used to estimate solid-fluid equilibrium. We consider two cases. First, when both the fluid and the solid phases are ideal solutions. This approximation is applied when the particle size ratio is close to unity. Second, where an ideal fluid mixture is in equilibrium with a pure solid. This approximation is applied when substitutionally disordered solid solutions are not stable with respect to phase separated pure solids. We compare the predictions of these simple approximations with those from an accurate molecular theory which we have recently developed for these systems. In particular we show that the phase diagram is most significantly influenced by the nonideality in the solid phase. Solid-fluid phase diagrams for hard sphere mixtures can be accurately predicted by assuming ideal solution behavior in the fluid phase. (C) 1997 American Institute of Physics. [S0021-9606(97)50741-0].