The precipitation reaction between aminotri(methylenephosphonic acid) (ATMP), a phosphonate used for scale prevention in high-water-volume industrial processes such as petroleum production, and calcium was systematically studied. By varying the precipitating conditions, three distinct precipitates formed:  a crystalline, sheetlike, 1:1 calcium−ATMP precipitate; an amorphous, spherical-shaped, 2:1 calcium−ATMP precipitate; and an amorphous, spherical-shaped, 3:1 calcium−ATMP precipitate. Corresponding batch dissolution experiments showed that as the precipitate calcium−ATMP molar ratio increased from 1:1 to 2:1 to 3:1, the rate of dissolution and the equilibrium solubility limit decreased significantly. The significance of these observations was evident when the release characteristics of each precipitate from porous media were studied as related to ATMP use in oil-recovery systems. The 3:1 calcium−ATMP precipitate was released from porous media in a much slower manner than the other two precipitates, strongly suggesting that the 3:1 precipitate is most suitable for use in oil recovery.