The influence of structurally related additives, namely N4-acetylsulfamerazine (NSMZ), sulfadiazine (SD) or sulfamethazine (SM), on the rate of the solvent-mediated polymorphic transformation (I --> II) of sulfamerazine in acetonitrile (ACN) at 24degreesC was studied. The transformation rate is controlled by the crystallization rate of the more stable Polymorph II. All three impurities exhibit inhibitory effects on the crystallization of Polymorph II and hence stabilize the metastable Polymorph I in ACN suspension. The rank order of the inhibitory effect (NSMZ much greater than SD > SM) is the same as the rank order of the binding energy of the impurity molecule to the surface of the host crystal. The relationship between the concentration of the impurity and the inhibitory effect was fitted to various models and was found to be best described by a model based on the Langmuir adsorption isotherm. (C) 2002 Published by Elsevier Science B.V.