The buffering capacity of casein micelle suspensions is a physico-chemical characteristic depending on the mineral and protein composition, on the interactions between these compounds and on the physicochemical environment. In this study, buffering capacity was maximum at pH about 5.2 and was mainly due to the acid-base properties of inorganic phosphate solubilised during acidification and organic phosphate from phosphoserine residues of caseins. Addition of calcium chloride to suspensions of casein micelles in water slightly increased buffering capacity, whereas addition of sodium phosphate or sodium citrate increased buffering capacity both qualitatively and quantitatively. These changes were due to intrinsic acid-base properties of the added compounds (citrate and phosphate) and to the physicochemical modifications of micellar caseins induced.