Sulfidation of zinc plates by H2S and H2S/H2O supercritical fluids at 400 degrees C has been studied. ZnS (wurtzite) nanoneedles with the diameter up to 100 nm and length up to 3 mu m are synthesized at zinc sulfidation by H2S, while the equiaxial ZnS (wurtzite) and ZnO (zincite) nanoparticles with the size up to 200 nm are formed in the presence of water. The catalytic effect of water on zinc sulfidation has been found. This effect is caused by the fact that the rate of ZnO nanoparticles synthesis in the reaction Zn + H2O = ZnO + H-2 is higher than that of ZnS nanoparticles in the reaction Zn + H2S = ZnS + H-2. Also, it consists in the regeneration of water by the reaction ZnO + H2S = ZnS + H2O and its subsequent participation in ZnO nanoparticles synthesis resulting in continuous increase in the ZnO contact surface with H2S. We established that the CS and CS2 impurities in hydrogen sulfide reacted with zinc oxide to form CO, CO2, and carbon deposits on the ZnO surface. (C) 2014 Elsevier B.V. All rights reserved.