Thermoelectric CoSb3-delta skutterudite was synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. Structural analysis carried out from synchrotron x-ray powder diffraction data reveals a significant Sb deficiency in this material. The introduction of point defects in the form of Sb vacancies distributed at random in the structure leads to an impressive reduction (> 50%) of the total thermal conductivity, kappa, which is one of the main ingredients of good thermoelectric materials. This suggests phonon scattering effects originated in the Sb defects, which drives to a better improvement in kappa than that achieved by the conventional strategy of filling the cages of the skutterudite structure with rare earths or other heavy cations. In parallel, changes in the electronic band structure caused by point variation of the stoichiometry produce an undesired increment in the electrical resistivity. Nevertheless, the low thermal conductivity combined with a high Seebeck coefficient (-434 mu VK-1 at 500 K) originates a relatively large figure of merit (ZT = 0.12 at 550 K) for CoSb2.90(2). (C) 2017 Author(s).