The effect of six different bi- and trivalent cations impurities on the metastable zone width for the growth of ammonium oxalate monohydrate crystals from aqueous solutions at 30degreesC is investigated as a function of impurity concentration c(i). It was observed that: (1) Mn(II), Co(II) and Ni(II) impurities lead to an increase in the value of the supersaturation ratio S-max corresponding to metastable zone width, the effect of Co(II) and Ni(II) being more pronounced than that of Mn(II), and (2) Cu(II), Fe(III) and Cr(III) impurities have practically no effect on Smax. The experimental data were analysed using the theory for heterogeneous three-dimensional nucleation and the concept of appearance of supersaturation barriers. Analysis of the observations revealed that: (1) the dependence Of Sax on the concentration ci of an impurity is in agreement with the predictions of the theory for heterogeneous three-dimensional nucleation in which adsorbed impurity particles block the active growth sites available on the surfaces of nuclei developing in the bulk solution, (2) the value of S-max is associated with the average distance between active adsorption sites, which is related with the average distance between adsorbed impurity molecules on the growing surface of the crystal, and (3) an impurity leads to an increase in the metastable zone width of ammonium oxalate solutions when the complex adsorbing on the growing surface is not very stable. (C) 2004 Elsevier B.V. All rights reserved.