Evidence is presented of mixing effects on the precipitation of calcium oxalate from aqueous solutions of calcium chloride dihydrate and sodium oxalate in both MSMPR and single-feed semi-batch precipitators. Experiments of precipitation were carried out in a pilot tank of 20 dm(3) volume stirred by three different impellers. The influence of stirring speed, stirrer type and reagent feed tube location on the product size and morphology was studied. Whereas weak effects of these operating parameters were noticed in the continuous operation mode, the semi-batch single-feed mode was shown to be more sensitive to mixing effects. Experimental results on crystal mean size during single-reed semi-batch experiments could be related to the temporal variance of the concentration field of a fluorescent inert tracer injected into the same feed location and tube in the tank which was then continuously operated. These variances were measured by laser sheet visualization and image processing techniques.