Bulk samples of MgxB2 with starting composition of 0.5 <= x <= 1.3 were prepared using a solid-state reaction route. Their structure and critical current density (J(c)) were investigated. The experimental results show that the starting composition MgxB2, as well as the sintering temperature, has a significant influence on J(c) and H-irr. The lattice parameter a increases with x and c shows a maximum value around x = 0.9. All the samples possess a J(c) value about 10(6) A/cm(2) in self-field for x > 0.5. However, at high magnetic field the Mg-deficient samples exhibit higher J(c) values. The highest irreversibility field of H-irr = 5.2 T at 20 K was reached for x = 0.8 with sintering temperature of 800 degrees C, which is 0.8 T higher than that of the stoichiometric MgB2 sample. It is suggested that the formation of MgB4 nanoparticles is responsible for the increase of H-irr and J(c). (c) 2005 Published by Elsevier B.V.