The rare-earth nickelates are among few perovskite oxides showing very sharp temperature-induced metal/ insulator transition. The synthesis and study of oxygen-deficient NdNiO3-delta and SmNiO3-(delta) are described in this paper. The results obtained indicate that the oxygen nonstoichiometry greatly influences transport properties of these compounds. The sharpness of metal/insulator transition strongly rises with decrease of oxygen deficiency without any noticeable change of transition temperature. The boundaries of oxygen and cation homogeneity ranges of Nd1-xNiO3-delta were determined for the first time. The thermogravimetric study was used to explain certain difficulties in oxidation of as-synthesized oxygen-deficient NdNiO3-delta samples. (C) 2004 Elsevier Ltd. All rights reserved.