A new method for the measurement of the concentration of humic substances (HS) in seawater is described. The peak obtained by differential cathodic sweep voltammetry after HS adsorption in the presence of traces of molybdenum(VI) can be used for quantitative determination. Measurements using various electrochemical techniques (ACP, CV, ADPV, DPP) would indicate that the film adsorbed onto the electrode is the result of the adsorption of the organic substance in a first step followed in a subsequent step by the formation of a surface complex (Mo(VI)-HS)adsrather than the direct adsorption of the dissolved complex. The electrochemical mechanism corresponds to a one-electron reduction of the adsorbed (Mo(VI)-HS)adscomplex to the corresponding (Mo(V)-HS)adscomplex.