Perovskite-type cobaltites LaCo1-xMgxO3 (0 < x <= 0.20) were synthesised by the liquid mix technique and structurally characterised by X-ray diffraction and neutron powder diffraction. This system can be regarded as LaCoO3-derived by means of partial substitution of trivalent cobalt ions by Mg2+. This doping is accompanied by the stabilization of the correspondent amount of Co4+ cations as it has been established from ICP, thermogravimetric and neutron diffraction results. The title materials behave as semiconductors up to 800 K. Above this temperature they show a transition to the metallic state. Magnetic susceptibility and magnetization measurements show weak ferromagnetic interactions at 5 K which has been interpreted taking into account disordered spin configurations for the cobalt cations. (C) 2009 Elsevier B.V. All rights reserved.