The dissociation of casein was studied after addition of polyphosphate that leads to calcium chelation, using light and X-ray scattering and turbidimetry. It is shown that the dissociation is a cooperative process; that is, a casein complex is either completely dissociated or remains largely intact. A systematic study was done of the dependence of the rate and extent of dissociation on the polyphosphate concentration and was found to be determined by the ratio between casein and polyphosphate. The structures of the casein complex and the small micellar particles formed after dissociation were compared. Additional experiments with a different chelatant (EDTA) gave similar results.