ZnS:Ir,Cl phosphors with an Ir content ranging from 0 to 4 x 10(-3) mol/mol Zn were synthesized by firing powder mixtures of ZnS, IrCl3, and flux. The crystal structure, morphology, luminescence properties, valence of Ir ion, and sulfur vacancy (V-S) in the samples were investigated. The X-ray diffraction measurements indicated that Ir was incorporated in ZnS without segregation up to 4 x 10(-4) mol/mol Zn. The photoluminescence spectra of the ZnS:Cl sample consisted of two peaks at 440 and 515 nm, originating from the self-activated (SA) center and V-S, respectively. On the other hand, a new emission band related to Ir did not appear in the spectra. It was found that the intensity of the SA emission increases while that of the V-S-related emission decreases remarkably with increasing Ir content. The X-ray absorption near-edge structure and electron spin resonance measurements suggested that the Ir incorporated as the trivalent ion in ZnS prevented the formation of V-S, leading to the improved photoluminescence property. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3541254] All rights reserved.