In groundwaters containing natural organic matter (NOM), mostly hu-mic substances (HS), it is expected that it plays a role on the behaviour of uranium in the environment. Another point is the actual effect of the competition with ma-jor cations and carbonate ions for U(VI)-NOM complexation. Our aim is to ac-quire experimental data for the U(VI)-HS complexation with two new different methodologies. The NICA-Donnan model is then used to predict uranium speci-ation in different groundwaters.