In order to investigate the role of firing atmosphere on the rate of transformation of a ZnS crystal, crystalline ZnS of hexagonal or cubic form was fired for 0.5–24 h at 800 and 1050 °C in each atmosphere of He, HCl, H2S, H2, O2, and their various mixtures, and the crystal structure of the fired ZnS was analyzed by x‐ray diffraction. At 800 °C, the transformation from hexagonal to cubic form occurred slightly in O2 and H2S, markedly in HCl, but hardly at all in He and H2 although NaCl was added. At 1050 °C, the transformation from cubic to hexagonal form occurred easily in all atmospheres except H2 and notably in HCl. The transformation occurred in H2 with added NaCl more easily than in pure HCl. Based on these results, it is concluded that the rate of transformation of a ZnS crystal is influenced both by the formation of defects, which result in the contact reaction between the atmosphere and the surface of the ZnS crystal, and by the bulk diffusion of anions (S2− and O2−).