Chemical scale inhibitors are commonly used to prevent or inhibit scale formation in production. The most economic treatment of scale inhibitor is normally via chemical squeeze. However, there is little agreement regarding the primary mechanism by which the threshold scale inhibitors are retained in producing oil or gas formations as a result of squeeze procedures. Recent advances in phosphonate/rock interaction research at the Rice University Brine Chemistry Consortium have significantly improved our knowledge of what controls inhibitor placement in the formation. It is commonly suggested that reservoir type determines how an inhibitor is retained in a formation. Our research suggests that the pill chemistry is also an important determinant for retention of carbonate reservoir. Acidic pills are mostly retained near the well bore while more neutralized pills move farther into the formation. Three calcium nitrilomethylenephosphonate (NTMP) solid phases, an amorphous phase and two crystalline Ca2.5HNTMP phases with pKsp = 22.6 and pKsp = 24.2, are particularly important toward inhibitor retention. The relative sizes of these solid phases formed is governed by the pill composition and acidity. These results can be explained by a solution phase controlled sequence of reactions. All of this information has been incorperated into a new squeeze design software program, SqueezeSoftPitzer.