The inversion between sphalerite and wurtzite in the system Zn-Fe-Ga-S was investigated by dry synthetic phase-equilibrium experiments. In the projection diagrams of the ZnS-FeS-GaS pseudoternary system at 900 degrees and 800 degrees C, the (Zn,Fe,Ga) (sub 1-x) S solid solution extends over an extensive region from the ZnS composition toward both FeS- and GaS- compositions. At 900 degrees C, this solid solution has the sphalerite structure along the ZnS-FeS join, consists of a mixture of sphalerite + wurtzite where there is a small amount of GaS component, and adopts the wurtzite structure where there is a greater proportion of the GaS and FeS components. A mixed sphalerite + wurtzite type occurs in the GaS-rich portion of the ZnS-GaS join and slightly extending toward the FeS composition. At 800 degrees C, there is a similar distribution of polymorphs within the solid-solution field, except that the mixed sphalerite + wurtzite field is more extensive and includes the entire ZnS-GaS join. Heating experiments show that the inversion from the sphalerite structure to the wurtzite structure for a sample having composition (ZnS) 70 (GaS) 30 occurs near 875 degrees C. Reaction rates of this inversion are slow in both directions; the inversion temperature depends on the bulk composition (especially the concentration of cations) and the sulfur fugacity.