We have studied the precipitation of calcium oxalate monohydrate (COM) from supersaturated saline solutions both as an in vitro model of the formation of human kidney stones and more generally as a model of particle-size enlargement during precipitation from solution. In any precipitation system there are two mechanisms for size enlargement : growth, by which is meant the deposition of ionic or molecular species on crystal surfaces, and aggregation, the process by which crystals collide, adhere to each other, and form new, stable particles. In this paper we report batch precipitation experiments conducted at constant initial supersaturation and varying ratios of calcium to oxalate ions. By comparison with a simple diffusion-reaction model we are able to draw substantial inferences on the processes that control aggregation. We conclude that at constant temperature and agitation, the aggregation rate constant depends only on the cementing rate, which in turn depends on the solution composition and an appropriately formulated Thiele modulus only. Thus, for the first time, it is possible to propose and evaluate a physico-chemical model for the kinetics of aggregation during precipitation from solution.