Mechanisms of flux action for the growth of ZnS phosphor particles have been investigated by using a sealed quartz capsule, instead of an unsealed crucible used in practical production process. No growth of ZnS particles is found in the melted sodium chloride phase. This result is in remarkable contrast to the classical model by Leverentz, in which melted NaCl acts as flux for the ZnS particle growth. It is revealed that the fluxing agent is the vaporized ZnCl2 gas, which is formed as a result of sequential chemical reactions. A growth mechanism of ZnS particles by the chlorine cycle via ZnCl2 and Cl-2 gases is proposed, with emphasis on the importance of growing spaces (voids) in the blend-mixture charged in a crucible. On the basis of the present results, it is stressed that further improvement is possible for the production process of ZnS phosphor powder. (c) 2005 Elsevier B.V. All ri-hts reserved.