Phosphonates are water treatment chemicals that are effectively utilized in many industrial processes as dispersants, bleaching agents, or scale and corrosion inhibitors. In many of these applications, the phosphonates are able to react with divalent cations such as calcium to form stable divalent cation-phosphonate precipitates. The focus of this paper is to define the conditions under which distinct calcium-phosphonate precipitates will form and to study how each of these precipitate’s unique chemical and physical properties govern the release of phosphonate from porous media. The phosphonate used in this study was (l-hydroxyethylidene)-1,1-diphosphonic acid (HEDP). By variation of the pH and calcium/HEDP molar ratio in solution, two distinct precipitates were formed : (1) a soluble, fibrous 1:1 calcium/HEDP precipitate; and (2) a less soluble, spherical 2:1 calcium/HEDP precipitate. Critical pH values that define the conditions under which each distinct precipitate forms were identified. Below the first critical pH value, the 1:1 precipitate formed, while above the second critical pH value, the 2:1 precipitate formed. Finally, coreflood and micromodel experiments showed that the release of 2:1 precipitate from porous media was significantly slower than that of 1:1 precipitate, suggesting that the 2:1 precipitate is better suited for phosphonate treatments in oil field applications. The release of a precipitate mixture (one which contains both distinct precipitates and has a calcium/HEDP molar ratio of 1.4:1) from a micromodel reconfirmed this phenomenon.