- Abstract The phase diagrams of binary alloys are known to vary with pressure. Within the framework of solution-type models, the variation is controlled by the pressure dependence of the elemental end members, which is relatively well known, and that of the excess interaction, which is unknown. We develop an exact thermodynamic relationship for the pressure dependence of the excess interaction in an alloy solution and relate it to the composition dependence of the density and sound velocity measured at ambient pressure. The effect of composition on the sound velocity and density was studied for the isomorphous binary alloy Bi Sb and the eutectic systems Bi Sn and Pb Sn, and the pressure dependence of the excess interaction was evaluated. On the basis of these results, the pressure dependence of these binary alloy phase diagrams was calculated and found to agree well with the experimental data in the literature.