ZnS nanoparticles have been synthesized using the amino acid histidine as a stabilizing agent. The syntheses were carried out by varying the starting pH and histidine/Zn2+ ratio. The as-prepared ZnS nanoparticles were characterized by various analytical techniques. The photocatalytic activity of the ZnS nanoparticles was determined by studying the degradation of methyl orange. The ZnS nanoparticles synthesized with 1:1 histidine/Zn2+ ratio and starting pH of 10.3 were found to exhibit the highest photocatalytic activity. Nearly 95% of methyl orange was degraded in 30 min of irradiation using the photocatalyst. Particle size was not the main factor in determining the photocatalytic activity of the ZnS nanoparticles. Fluorescence lifetime measurements indicated that photocatalytic activity of the ZnS nanoparticles was enhanced with increase in their fluorescence lifetime. (C) 2013 Elsevier B.V. All rights reserved.