A parameter, 0 less than or equal to beta less than or equal to1, which characterizes the back-diffusion of solute in the mushy region of a solidifying alloy is introduced. The parameter is general and no specific assumptions on the nature of the solidification are made in arriving at its form. Use of the parameter in the analysis of binary alloy microsegregation controlled by a parabolic solid growth results in a specific microsegregation model. At high solid fractions, predictions from this model significantly outperform alternative models in the literature.