Nickel metal nanoparticles supported on graphitized carbon have been reacted with H2S/H-2 gas mixtures to produce catalytic nickel sulfide nanoparticles. By varying the conditions of H2S concentration, reaction time, and reaction temperature, nanoparticles of beta-Ni3S2, Ni9S8, Ni7S6, and beta-NiS have been selectively produced. The average particle size was found to increase with conversion of the nickel metal particles to nickel sulfide particles with values that agreed with those obtained from density calculations. The phases formed were characterized by PXRD, HRTEM, and EDX and at 300 degreesC were those predicted from previous experiments on bulk samples. However, at 600 degreesC, the low-temperature phases-beta-Ni3S2, Ni9S8, and Ni7S6-formed rather then the higher temperature phases adopted by bulk samples at this temperature.