Aggregation is an important crystal size enlargement mechanism during precipitation and the rate of aggregation is affected by the Various process conditions such as solution composition and the hydrodynamics of the system. In this paper, an investigation of the effect of hydrodynamics as well as solution composition on the aggregation of calcium oxalate monohydrate crystals is reported using a novel Poiseuille Flow Crystalliser (PFC). The well-characterised hydrodynamics of the PFC allows the prediction of collision rates using Smoluchowski's theory and therefore the quantification of the efficiency of aggregation. At low shear rates, i.e. in the high-efficiency regime, it is found that the aggregation rate constant varies linearly with shear rate providing good validation for Smoluchowski's theory. Further, at increasing shear rates a maximum in the value of the aggregation rate constant is observed confirming the disruptive role of fluid shear rate in impeding aggregation. In the investigation of the effect of solution composition on aggregation, it is found that the aggregation rate constant increases with the growth rate, but is independent of the species ion ratio. Finally, we find that dimensionless strength is a powerful correlating variable in determining aggregation efficiency.