In this work, a general multidimensional population balance (PB) model is developed to predict the coating volume distribution on polydisperse particles as a function of time during particle coating in a paddle mixer. The model adopts a compartmental approach to account for coating variation caused by particle flow heterogeneity. Simulations show that for a realistic range of seed particle size polydispersity and coating mass applied, the coating volume distribution depends on the growth rate exponent and seed particle size distribute on, with the coating volume coefficient of variance (CoV) approaching an asymptotic value as the coating-to-particle volume ratio increases. These effects cannot be predicted by simpler one-dimensional models. However, the full two-dimensional PB and simpler one-dimensional models such as Mann's equation do predict similar sensitivity of coating volume CoV to the variation in the compartment model parameters, i.e., to changes in the particle mixing behavior in the vessel. (c) 2011 American Institute of Chemical Engineers AIChE J, 2012