The adsorption of citrate and phosphocitrate on calcium oxalate monohydrate (COM) surfaces has been measured in saturated solutions of calcium oxalate monohydrate at 37°C. In separate adsorption experiments, the uptake of phosphocitrate was markedly greater than that of citrate. When both additives were present, the adsorption of phosphocitrate was further increased. In constant composition studies of the crystallization of CON from supersaturated solution, phosphocitrate was more effective as an inhibitor than was citrate.