Rotational tunnelling and librations of methyl groups in bis(trimethyltin) selenate dihydrate, [(CH 3 ) 3 Sn] 2 SeO 4 · 2 H 2 O, have been studied at low temperature using inelastic neutron scattering (INS) technique. Three tunnel transitions at 1.31 μeV, 3.47 μeV, and 21.0 μeV of equal intensity were recorded. In the energy range of phonons a broad, slightly structured intensity distribution is observed. An attempt is made to identify and assign librational modes. The crystal structure of the title compound was determined by single crystal X-ray diffraction. [(CH 3 ) 3 Sn] 2 SeO 4 · 2 H 2 O crystallizes orthorhombic, space group D 14 2h -Pbcn with 4 molecules in the unit cell. The lattice constants at room temperature are a = 1142.9(3) pm, b = 1304.6(4) pm, and c = 1084.6(3) pm. According to the molecular symmetry, the two "(CH 3 ) 3 Sn(H 2 O)" units in molecule are crystallographically equivalent, but the three methyl groups bound to one Sn-atom are inequivalent. This result coincides well with the observation of three tunnel transitions in INS. A comparison with the related compound, bis(trimethyltin) sulfate dihydrate, is presented. The existence of hydrogen bonding and the role of intermolecular interactions on rotational tunnelling are discussed.