Isoelectric precipitation of sunflower protein was carried out in a 273 mi MSMPR precipitator. Experimental results showed a bimodal particle-size distribution (PSD) of protein particles when the solids concentration or the mean residence time was low. Increasing the solids concentration and the mean residence time transformed the bimodal PSD to a unimodal PSD. Protein particle growth by turbulent collision mechanism and breakage by shear mechanism were modelled using an approach similar to Glatz et al. A.I.Ch.E. J. 32, 1196-1204 (1986). The model results showed that the breakage of large aggregates results in the birth of two daughter fragments. Also al high solids concentrations the particle growth rate was linear with respect to particle size. At low solids concentrations the growth rate constant was larger than the breakage rate constant and vice Del sa at high solids concentrations.