The selective precipitation of ␣-lactalbumin (␣-LA) at a pH around its isoelectric point (4.2) under heat treatment is the basis for a fractionation process of whey proteins. In these conditions, -lactoglobulin remains soluble, whereas bovine serum albumin and immunoglobulins co-precipitate. Knowledge of the mechanism governing the ␣-LA precipitation influences the choice of operating conditions and enables optimization of the fractionation process. ␣-LA is a calcium metallo-protein and its isoelectric precipitation is governed by the protein-calcium complexation equilibrium. Citrate, a sequestrant of calcium, decreases the free calcium concentration and displaces the precipitation phenomenon to a lower temperature range. A study of the effect of citrate on the precipitation phenomena of whey proteins is presented. Whatever the citrate content, precipitation curves for bovine serum albumin (BSA) and ␣-LA intersect at a temperature around 45°C. For a temperature of heat treatment lower than 40°C, a selective enrichment in ␣-LA of the precipitated phase is observed. As addition of citrate leads to high ␣-LA precipitated fractions at a temperature around 35°C, the precipitation step may be performed at this temperature. It results in a reduced heat denaturation of whey proteins and in a higher ␣-LA purity in the precipitated fraction.