A new physicochemical model of metal complexation by humic substances is described. The model takes into account the effects of double layer relaxation, chemical bond formation, and the increase in entropy associated with metal dehydration. In the case of the double layer thermodynamics, mathematical models, which may be used to calculate the enthalpic, entropic and total free energy contributions to the complexation reaction are formulated. The contribution of chemical bond formation to the enthalpy of reaction is investigated by comparison with simple ligand reaction enthalpies. In the case of the entropic contribution of metal dehydration, partial molar entropies were found to give the best measure. Binding enthalpies and entropies for Eu3+ and UO22+ with Aldrich humic acid have been determined: ΔH(Eu)=+36 kJ mol−1; ΔS(Eu)=+276 J K−1 mol−1; ΔH(UO22+)=+62 kJ mol−1; ΔS(UO22+)=+62 J K−1 mol−1. In addition, the effect of ionic strength on the binding of Cu and Cd has been studied electrochemically. Using these data, combined with data from the literature, the model has been tested, both qualitatively and quantitatively. The suitability of the model for the prediction of unknown humic/metal complexation enthalpies, entropies and total binding strengths is discussed.