The influence of impurities on the crystallization kinetics of ammonium sulfate was investigated. MSM PR experiments were conducted with the impurities aluminum sulfate and the azo dyes amaranth and fuchsine. Nucleation and growth rates as well as mean crystal sizes were related to the supersaturation sigma and the width of the metastable zone. It was found that all impurity levels in the system reduce kinetic coefficients for crystal growth and suppress nucleation by adsorption on the crystal surfaces. An increase of supersaturation and metastable zone width compensates for this reduction at low impurity concentrations and allows the growth of larger crystals compared to the pure system. At high impurity concentrations and increasing surface coverage of the crystals, supersaturation rises faster than metastable zone width. causing an increase in nucleation rates and a higher fines content in the product compared to the pure system. A similar interdependence between impurity concentration, crystal size and supersaturation was found for other systems not reported here. The observations made can be explained in terms of adsorption equilibria of the impurities on the crystals. This seemingly general relationship allows the adjustment of crystal sizes in crystallization processes by control of impurity concentrations. A second paper will discuss the changes in crystal morphology in greater depth (Kuch ct al.. 2000).