The experimental results of a study of the effect of concentration of Cu(II) ions and solution supersaturation on growth rates of ammonium oxalate monohydrate crystals, impurity segregation coefficient and sites of incorporation of cations in the crystal lattice are described and discussed. It is found that the effects of impurities on growth kinetics and the value of the segregation coefficient are related processes and involve adsorption of impurity particles on growing surfaces. Examination of EPR spectra shows that high incorporations of Cu(II) ions in the crystal lattice favour the substitution of ammonium sites while at low incorporations copper(II) ions enter the lattice both in interstitial and substitutional lattice positions.