We have investigated the double perovskites Ca2MSbO6 (M = Mn, Fe) that have been prepared by solid-state reaction (M = Fe) and wet chemistry procedures (M = Mn). The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the crystal structures are orthorhombic (space group Pbnm) with complete disorder of M and Sb cations, so the formula should be rewritten as Ca(M0.5Sb0.5)O-3. Due to this disorder no evidences of Jahn-Teller distortion can be observed in the MnO6 octahedra of Ca(Mn0.5Sb0.5)O-3, in contrast with the ordered double perovskite Sr2MnSbO6. Ca(Fe0.5Sb0.5)O-3 behaves as an antiferromagnet with an ordered magnetic moment for Fe3+ of 1.53(4)mu(B) and a propagation vector k = 0, as investigated by low-temperature NPD. The antiferromagnetic ordering is a result of the high degree of Fe/Sb anti-site disorder of the sample, which originates the spontaneous formation of Fe-rich islands, characterized by the presence of strong Fe-O-Fe antiferromagnetic couplings with enough long-range coherence to produce a magnetic contribution perceptible by NPD. By contrast, the magnetic structure of Ca(Mn0.5Sb0.5)O-3 cannot be observed by low-temperature NPD because the magnitude of the ordered magnetic moments is below the detection threshold for neutrons. (C) 2010 Elsevier Ltd. All rights reserved.