To analyse the properties of humic and fulvic acid standards (Suwannee River and Lauretian River) concerning their relation towards trace metals (Cd, Pb) and protons, several potentiometric and voltametric titrations have been carried out. The experimental results were achieved by logarithmic addition of standard solutions prior to mathematical treatment by new calculation software that was developed in our laboratory. PROSECE (Programme d'Optimisation et de SpEciation Chimique dans l'Environnement) defines a discreet model based on a so-called “chimio-type”, representing the characteristics of the analysed natural organic matter (NOM). This “chimio-type” consists of three different types of “quasi-particles” (Sposito, 1981), which allows to model the acid sites (6 “quasi-particles” of type I), convertible complexing sites (4 of type II) and specific complexing sites (2 of type III) inside the NOM. Optimised complexing parameters (e.g. concentration, complexation- and acid-constants of these “quasi-particles”) allow predicting the properties of NOM related to trace metals and to the phenomenon of metal/metal and metal/proton competition. These “quasi-dynamic' parameters can be integrated easily in a contaminants transport model like SiAM3D-MOCO (IFREMER).