Neutron diffraction (ND) Rietveld refinements at 3 K and ab initio density functional theory (DFT) calculations were performed on Ba6Co6ClO16-x, a new mixed valence cobalt oxychloride. The experimental data coincide with the theoretical ones to indicate that an antiferromagnetic ordering occurs. The refined magnetic structure displays antiferromagnetic Co2O7 tetrahedral dimers with a moment collinear to the c-axis, of 2.8 mu(B)/Co atom (ND), the DFT giving 2.3 mu(B)/Co. These results suggest that tetrahedral Co4+, in the intermediate spin configuration, are interconnected by tetrameric Co3+ octahedral units which are predicted by DFT to arrange ferromagnetically through delocalized electrons, in good agreement with ND. (c) 2006 Elsevier B.V. All rights reserved.