The solvation of mercury(II) complexes and ions has been studied by XAFS methods and compared to the corresponding thallium(III) species. Analyses of Hg L(III) edge extended X-ray absorption fine structure (EXAFS) spectra gave the distances 2.29(2) and 2.31(2) angstrom for HgCl2 in aqueous and dimethyl sulfoxide solutions, respectively, 2.46(2) angstrom for solid HgBr2 and 2.42(2) angstrom for HgBr2 in aqueous solution; for Hg(CN)2 in aqueous solution Hg-C = 2.04(2) angstrom and Hg-N = 3.18 (3) angstrom. The weakness of the EXAFS signals observed of the solvated Hg2+ ion in e.g. pyridine, acetonitrile, and aqueous solutions are interpreted as being due to dynamic distortions of the first solvation shell by second-order Jahn-Teller effects. The pre-edge transitions in the X-ray absorption near-edge structure (XANES) region for mercury(II) and thallium(III) complexes have been used to distinguish between different coordination geometries of the solvated species. Theoretical ab initio calculations have been performed on the structures of the mercury(II) and thallium(III) dihalide and dicyanide complexes, in order to compare the effects of differences in bonding and hydration, and also on the valence shell energy levels, to assist assignments of pre-edge features in the XANES spectra. Relativistic effective core potentials (ECP) were constructed, both for the ground-state mercury and thallium atoms and for the 2p ionized state, and used in calculations at the MCPF level of bond lengths and relative energy differences. The first pre-edge peak found for all complexes in their XANES spectra was assigned to a (2p) --> SIGMA(g)+ (approximately Hg 6s) excitation, with the splitting of the pre-peak for Hg(CN)2 possibly due to (2p) --> PI*(C-N) at ca. 3.4 eV higher energy. Multiple scattering resonances have been discussed for the CN ligands. Comparisons of calculated and experimental bond lengths of the mercury(II) and thallium(III) dichloride and dicyanide complexes revealed unexpectedly short bond lengths for the mercury(II) complexes, which have been discussed in terms of weaker solvation and stronger bonding. The bonding in the Hg(CN)2 and Tl(CN)2+ complexes were analyzed by theoretical calculations using a constrained space orbital variation (CSOV) technique, showing significant contributions of back-donation particularly in the Hg-CN bonds. The trends of the force constants from vibrational spectra are consistent with this picture and show stronger and shorter M-C bonds but also stronger C-N bonds in the Hg(CN)2 complex than in the Tl(CN)2+ complex.