Samples of the complex osmates Ca2Os2O7, Tl2Os2O7 − x and Pb2Os2O7 − x have been obtained by direct reaction of the component oxides in sealed tubes. The structures and compositions of these materials have been investigated by powder neutron and X-ray diffraction at room temperature and through variable temperature studies. Ca2Os2O7 adopts a stoichiometric, orthorhombic weberite structure but the distortion of the OsO6 octahedra and eightfold Ca–O co-ordinations decrease with increasing temperature indicating that this material lies close to the weberite/pyrochlore phase boundary. The lead and thallium phases adopt oxygen deficient, cubic pyrochlore structures under the synthesis conditions employed though no evidence was found of cation non-stoichiometry. Preliminary measurements of the electronic properties of weberite Ca2Os2O7 are also reported and discussed in comparison with the lead, thallium, cadmium and mercury osmate pyrochlores.