Benzoic acid has been crystallized by mixing sodium benzoate and hydrochloric acid in a T-mixer. The suspension is collected in an agitated crystallizer, and aging of the solid product is evaluated by microscopy. In addition, the kinetics of aging is examined by population balance modeling and nonlinear parameter estimation. Microscopic observations reveal that the crystal shape is quite irregular immediately after the T-mixer crystallizer, but during the aging the crystals turn into nice well-shaped platelets. A model on Ostwald-type ripening gives a fairly good description of the evolution of the crystal size distribution during the aging, provided that the observed changes in crystal shape are accounted for in the model.