Aggregation is an important size enlargement mechanism in various precipitating systems. Unlike the aggregation of colloidal particles in ionic solutions which has been extensively studied, aggregation during precipitation from supersaturated solutions is less well understood. In this paper experimental data from the batch precipitation of calcium oxalate monohydrate are used to show how the aggregation rate depends on solution composition. A discretized population balance method is used to extract the growth and aggregation rates from experimental particle size distributions collected at regular time intervals. It is found that for all the experimental conditions considered, the measured growth rate may be described by a kinetic equation which is second order in a relative supersaturation, regardless of the solution composition, that is the calcium to oxalate ion ratio. Further, it is demonstrated that the rate of aggregation exhibits a more complex dependence on solution composition than the growth rate. A model is then proposed based on the diffusion of solute ions to a cementing site, which despite a number of simplifying assumptions is able to explain the experimentally observed behaviour of the aggregation rate constant.