A method is presented for determining the composition of the material being instantaneously abstracted from the liquid during equilibrium crystallization of a solid solution in a binary system. It shows that, except when the solidus and liquidus curves are straight and parallel, the composition of the material being instantaneously abstracted from the liquid does not lie on the straight line joining the composition of the first crystals and last liquid for that bulk composition. Despite this, the latter straight line is a reasonable approximation to the actual curve.