A smooth relationship is obtained when the excess free energy of mixing parameter, Bs, is plotted against the relative difference in the lattice parameters of the two components of a number of miscible III-V pseudobinary semiconductor alloys. Extrapolation of this plot to values of the size disparity in systems for which experimental phase diagram inlormation is lacking gives estimates of Bs sufficiently large to predict miscibility gaps in the AlSb-AlAs, InSb-InP, GaSb-GaP, and AlSb-AlP systems, when the thermodynamic criterion for phase separation, Bs > 2RTc, is employed. A cutoff point for miscibility across the entire phase diagram appears to occur at a relative lattice parameter difference of about 7.5%, the value for GaSb-GaAs. This same relative difference is found to divide miscible from immiscible II-VI zinc blende alloys as well. © 1974, The Electrochemical Society, Inc. All rights reserved.