Complexing affinities of a concentrated marine dissolved organic matter (DOC content of 1.2 mmolC L-1) towards major and trace cations were studied by potentiometric and voltammetric titration techniques. A 2-steps protocol, involving nanofiltration and reverse osmosis, was applied for organic matter pre-concentration. A homewritten software, named PROSECE, was used for fitting of all experimental data in order to obtain complexation parameters. Four distinct classes of acidic sites with pKa of 3.6, 4.8, 8.6 and 12 were resolved according to potentiometric titration experiment. A total acidic sites density of 446.4 meq/molC was estimated, with a majority of carboxylic-like sites (60% of the acidic sites). Pseudopolarography of concentrated seawater sample with increasing concentration of copper ions identified three classes of copper complexes (1) labile – with reduction wave at ~ -0.2 V, (2) inert – with reduction wave at ~-1.4 V and (3) strong inert non-eletroactive (Figure 1). Simultaneous competition between copper, calcium and proton revealed the presence of two classes of binding sites forming inert complexes. These sites were of phenolic-like type with respective pKa of 8.6 and 8.2. The first class of binding sites was more specific to copper (logKCuL of 9.9 and logKCaL of 2.5), whereas stronger competition between copper and calcium occurred for the second class of sites (logKCuL of 6.9 and logKCaL of 5.5). The site density of second class was about six time higher than that of first class (10.25 meq/molC compared to 1.72 meq/molC), which, in summary, is only small part of the total acidic site density. The validity of sample preconcentration and data treatment was confirmed by very good accordance of the experimental titration data measured in non-concentrated seawater sample (DOC content of 0.09 mmolC L-1) with the data predicted by MINEQL simulation using complexing parameters obtained in the concentrated seawater sample. Furthermore, this comparison highlighted that the applied concentration protocol had negligible influence on organic matter properties when considering copper complexation.