The generally accepted view that phosphonate derivatives are more potent than the analogous carboxylates as crystal growth modifiers for barium sulfate was systematically studied by using trifunctional molecules varying from the triphosphonate through to the analogous tricarboxylate; the results suggest that predictions based on simple structural features should be made with caution. Control of particle morphology is a desired feature in many industrial applications 1-3 . Much research has focussed on being able to 'tailor-make' or design additives which affect particle morphology in a predicted way 4,5 . Barium sulfate has often been used as a model compound since it is easy to control the precipitating conditions and it is an industrially important scale forming compound. Previous investigations have been conducted which assess a systematic series of molecules [6][7][8][9][10][11][12][13] in an effort to relate crystallization inhibition to molecular structure. In these cases, such work has involved varying the structure of a molecule containing varying numbers of a specific functional group. Our approach has been to substitute one functional group with another. This minimises changes in the overall additive structure. It was found by Black et al. 7 that at least two phosphonate groups are required for inhibition (with a 3 atom chain between the phosphonate groups) and that small carboxylate molecules are ineffective as inhibitors and/or morphology modifiers 7 (polymers such as polyacrylates are of course used commercially as scale inhibitors since the relatively weak carboxylate-crystal surface interaction is more than compensated for by the large number of interactions available 14 ). Very recently, however, aminocarboxylates have been shown to have an effect on barium sulfate morphology 15 . These experiments were conducted at very high pHs (10-12) and at high carboxylate to barium ratios resulting in chelation of the barium in solution, which could explain why their results differ from those of Black et al. 7 The effect of chain length was studied by Bromley et al. 6 , where a series of tetraphosphonate molecules of differing carbon chain length (6-11 carbons) was investigated where two methylenephosphonate molecules were attached via a nitrilo group to the carbon chain at each end. The greatest inhibition occurred when the link between the two sets of phosphonate groups was greater than 6 Å and allowed at least two of the four phosphonate groups on the molecule to adsorb onto the surface in place of sulfate groups. Thus, although greater than two phosphonates are present on the organic molecule, adsorption is assumed to require only two of these groups. Using these principles, inhibitors have been designed and shown to be effective 6,9,16 . We decided to test these conclusions by systematically altering the functional group in a series of molecules with a consistent structural framework. Thus, we set out to investigate the inhibiting efficacy of a molecule containing a decreasing number of phosphonate grou...