Octacalcium phosphate (OCP) is an important P solid phase in geochemical and biological systems and has been recognized as a precursor phase to the formation of thermodynamically more stable hydroxyapatite (HAP). Metastability of OCP with respect to HAP may be explained by precipitation kinetics and the influence of dissolved organic C (DOC) on crystal growth. Octacalcium phosphate precipitation was measured at pH 6.0 and 25°C in the absence and presence of organic acids commonly found in natural waters and soil solutions using a seeded crystal growth constant composition method. Rate constants for OCP precipitation were calculated from the following expression: Rate = kS(IAP 1/8 - K sp1/8) n, where k is the rate constant (L 7 mol -6 m -2 s -1), S is OCP seed crystal surface area (m 2 L -1), IAP = ion activity product, Ksp = OCP solubility constant (mol 8 L -8), and n is the rate reaction order. The rate constant for OCP precipitation in the absence of organic acids was 10 34.93·L 7 mol -6 m -2 s -1. Humic, fulvic, tannic, and citric acids were added to OCP crystal growth experiments at total soluble (C TS) C levels ranging from 20 μM to 2 mM. Inhibition of OCP precipitation was nearly complete (99% ) in the presence of 1.0 mM C TS as humic acid. At the same level of C TS, OCP precipitation was inhibited by 97,88, and 68% in the presence of fulvic, citric, and tannic acids, respectively. Inhibition of precipitation is caused by adsorption of organic acids onto OCP surfaces blocking active crystal growth sites. The ability of organic acids to inhibit OCP crystal growth is related to their hydrophobicity, functional group content, size, geometry, and orientation on the crystal surface. Precipitation kinetics and crystal growth inhibition by organic acids may explain the metastability of dicalcium phosphate dihydrate (DCPD) and OCP with respect to thermodynamically more stable HAP often observed in geochemical environments.