The thermodynamics of binary alloy phase diagrams of ascending and minimum types is shown to lead to a rather direct relation between the ratio of the slopes of the liquidus and the solidus curves, at a given temperature T, and the ratio <(δc)21(δc)2sof the concentration fluctuations in the liquid and the solid solutions. The latent heats of the two pure components also enter in a direct manner, as could have been anticipated from the results of ideal solution theory