The paper presents and discusses the chemical affinity of two different forms of natural organic matter (e.g. in solid (SOM) and soluble form (DOM)) for Cu(2+) and Eu(3+) ions. In the this study SOM is represented by olive cake and DOM by its hydrophilic extracts. The investigations were performed in aqueous 0.1M NaClO(4) solutions at pH 6, 23 degrees C and under atmospheric conditions by means of potentiometry using a copper ion selective electrode and competitions reactions between the two metal ions. The experimental data were evaluated according to an "operational" approach, which is based on the charge neutralization model and the Scatchard approach. Evaluation of the data resulted in the determination of conditional formation constants (beta(*)), which are a measure for the chemical affinity of natural organic matter for a metal ion. For the olive cake metal ion binding the values of log beta(*) for Cu(II) and Eu(III) were evaluated to be 5.1+/-0.3 and 5.4+/-0.5, respectively, whereas for the hydrophilic extracts the corresponding values for the Cu(II) and Eu(III) complexes were evaluated to be 5.3+/-0.3 and 6.3+/-0.5, respectively. The beta(*) values show that natural organic matter in the two different forms presents almost similar affinity for the Cu(2+) ion, whereas the affinity of DOM for the Eu(3+) ion is significantly higher than the corresponding affinity of SOM. This is ascribed to the higher flexibility of DOM, which allows better coordination of its active sites around the Eu(3+) ion.