Extended X-ray absorption fine structure measurements have been conducted on solutions of Pu(IV) with varying concentrations of sulfate anion in aqueous solution. Data support the periodic progression to primarily bidentate sulfate coordination about An(IV) centers when moving heavier in the actinides. This observation is attributed to the increase in Lewis acidity of the tetravalent actinides across the series, a consequence of the filling of the 5f orbitals and the actinide contraction. Though the changes in the chemical formation energies of the An(IV) complexes between Th and Pu are small, it is these small energetic differences that are critical to understanding how to effect more efficient and effective chemical separations among isovalent metal ions.