Tracer crystals are developed to study the agglomeration, growth and residence time behavior of gibbsite crystals, an industrially important polymorph of Al(OH)(3), in continuous and batch precipitation. Satisfactory tracer crystals contain detectable amounts of Zn and have the crystal structure of gibbsite. The tracer crystals behave in the same manner during precipitation as the nontracer Al(OH)(3) crystals. The application of the tracer crystals for investigating, both batch and continuous, seeded Al(OH)(3) precipitation is demonstrated. The tracer experiments provide information on the growth and agglomeration kinetics of different size fractions. The tracers are a useful tool for the quantitative investigation of solid-phase residence time distributions within a crystallizer.