The compositions, unit cell parameters, and T-c of metal-dielectric transition were independently studied in hexagonal NiS crystals with the NiAs-type structure. Melt-grown single crystals were used in the study. The dependences of a(0), c(0), and T-c on the composition of NiS are discussed. The highest values of these parameters are measured in the crystals grown in the horizontal furnace. These crystals have the smallest deviations from the NiS stoichiometry. We have found also that the XRD measurement of the unit cell parameters is the most fast and convenient method for composition and T-c evaluation in hexagonal NiS. (C) 2003 Elsevier Science B.V. All rights reserved.