Growth kinetic of (10 0) typefaces of KDP crystals in impure solutions were measured and analyzed. The dual action of impurities as promoters of nucleation at higher supersaturations and as stopper of the spreading layer on the crystal surface at smaller supersaturations is underlined. 2D heterogeneous nucleation mechanism was found to be dominant at higher supersaturations, where the surface coverage of ad-molecules theta similar to 10(-6) is of the same order of magnitude as the impurity concentration in solution. At smaller supersaturations the dislocation mechanism of growth is severely retarded by the stopper action of impurities. In the transient region, at supersaturations higher than the critical ones, the stoppers action of impurities prevails, substantially decreasing the growth rates. The critical coverage theta* similar to 10(-3) at the limit of the "dead" growth zone, suggest large variations of the segregation coefficient with supersaturation for impurities having large energy of adsorption in the surface layer. (C) 2003 Elsevier B.V. All rights reserved.