Experiments on semibatch reaction crystallization of benzoic acid are reported. The conditions in an agitated tank are simulated by a loop reactor by which feed point mixing conditions can be controlled separately from the macroscale circulation rate. Hydrochloric acid is fed into a circulating solution of sodium benzoate and the influence of macromixing, mesomixing and micromixing on the product crystal mean size is evaluated. The product mean size increases with increasing circulation rate in the loop, with increasing feed point mixing intensity, with decreasing feed rate and with decreasing feed pipe diameter. Increased mixing intensity on any level leads to larger product crystals, but especially the rate of mesomixing is of importance. The influence of the feed pipe diameter is opposite to predictions by available theories and cannot be explained by backmixing into the feeding pipe. All results can be correlated quite well against a dimensionless mixing efficiency defined as the ratio of the reactant feeding time to the mixing time. The mixing time is the sum of the time constants for mesomixing and micromixing. A new mesomixing time constant is defined as being proportional to the ratio of the feed pipe diameter and the velocity of the bulk flow passing the feed pipe.