The atomic and electronic structure of tin tungstates, alpha-SnWO4, alpha-Sn1.03W0.99O4 and beta-SnWO4, was studied by the W L-3-edge X-ray absorption spectroscopy and first-principles linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange-correlation density functional (DFF)/Hartree-Fock (HF) scheme. It was found that the crystal structure of both alpha-phases is built up of strongly distorted WO6 octahedra, whereas that of beta-SnWO4 is composed of nearly regular WO4 tetrahedra. In addition, there are distorted SnO6 octahedra in both alpha- and beta-phases. The metal-oxygen octahedra distortion is explained by the second-order Jahn-Teller effect. The influence of pressure on the structure of alpha-SnWO4 and beta-SnWO4 was studied in detail based on the calculated equations of state. The compressibility of beta-SnWO4 was found to be larger than that of alpha-SnWO4. The existence of the insulator-to-metal transition was theoretically predicted in alpha-SnWO4 at about 16 GPa and was explained by a symmetrization of metal-oxygen octahedra leading to a strong interaction of Sn 5s, W 5d and O 2p states and closing of band gap. (C) 2014 Elsevier B.V. All rights reserved.