Guidelines for the design of fractional crystallization processes to separate two- and three-solute mixtures are presented. By using solvent addition/removal stream combination, and cooling/heating, these processes bypass regions of multiple saturation in the phase diagram and recover pure solutes. Design equations are formulated, and the constraints on the design variables are identified. Also included is a discussion of the effect on recycle flows of changes in the design variables and an estimate of the cost of a factional crystallization separation.