Complexation reaction between Yb3+ and human serum albumin is examined using isothermal titration calorimetry (ITC). The extension solvation theory was used to reproduce the enthalpies of HAS + Yb3+ interactions over the whole range of Yb3+ concentrations. The binding parameters recovered from this model were attributed to the structural change of HSA. The results show that Yb3+ ions bind to HSA with three equivalent affinity sites. It was found that in the high concentrations of the ytterbium ions, the HSA structure was destabilized.