An optical flow-cell assay was developed which allowed for simultaneous measurement of calcite and aragonite growth under conditions of constant concentrations of constituent ions and growth-modifying additives. Using this assay, it was determined that L-Citrate and D,L-isocitrate selectively inhibited the growth of aragonite crystal seeds, while having little or no effect on calcite growth rates at supersaturations in the range of sigma = 2.5 +/- 0.4. Tricarballylate, which differs from citrate and isocitrate by the absence of a hydroxide functionality, had very little effect on calcite or aragonite growth. For citrate on aragonite, an effective surface binding constant of K = 2.5 +/- 0.6 x 10(5) M-1 was derived from the kinetic data; for isocitrate, K = 2.9 +/- 0.8 x 10(5) M-1. The similarity of these effective binding constants argues against precise stereochemical recognition of the aragonite surface. (C) 2008 Elsevier B.V. All rights reserved.