Previous analyses of the liquidus and solidus lines in III–V pseudobinary systems are combined and extended. (i) The ideal, strictly regular, athermal, and quasiregular models are investigated for each phase. (ii) A quantitative measure, σ(°C) of a fit is defined and determined. (iii) All sets of values for the parameters of a model giving a value of σ less than a critical value (7°–16°C) characterizing the experimental accuracy are determined and treated on an equal footing. It is found that the strictly regular liquid‐quasiregular solid model is the simplest model giving satisfactory fits for all of the seven systems chosen as the best defined experimentally. Moreover these fits are obtained over a range of values of the three model parameters. As a consequence, thermodynamic properties of the liquid are not closely specified by the analysis. In contrast, for the solid at one temperature, , we find , where the y's are mole fractions and and are determined from the fits: and are correlated to the properties of the III–V components, and of the average of their melting points while, within is proportional to the sixth power of the fractional difference in their lattice parameters. Three of four other systems with data satisfy these correlations. Seven others with no data are discussed. The stability of a quasiregular phase is discussed and the conditions for the formation of normal and “inverted” miscibility gaps established.