CuGaS~ and ZnS form solid solutions over the entire range with some evidence for compound formation at 33 1/3 mole % CuGaS~. Emission under 3650.~ excitation shifts from green to red with increase in CuGaS~ concentration. Samples with more than 20 % CuGaS2 do not luminesce at room temperature but, at --195~ fairly bright emission is observed with up to 95% CuGaS2. AgGaS~ is soluble in ZnS to about 5-10 mole %, above which separation of AgeS is observed. Emission under 3650A excitation shifts from the blue (0.01% AgGaS2) to yellow (10% AgGaS~).The ternary sulfides, CuGaS~ and AgGaS~, have the chalcopyrite structure which is closely related to the zinc blende structure of cubic ZnS (1). In these compounds, two different atomic species occupy equivalent lattice sites on the zinc blende lattice. In CuGaS~, a z 5.34A and c = 10.47A. (c/a = 1.96) while in AgGaS2, a = 5.74A and c = 10.26A (c/a = 1.79) (2).Because of the similarities in structure, unit cell dimensions, and bond type, the ternary sulfides may form solid solutions with ZnS. In such solid solutions, the ternary compounds would not only change the unit cell dimension of ZnS, but may also affect the observed luminescence in ZnS: Cu,Ga and ZnS:Ag,Ga phosphors. Cu and Ag are activators and Ga is a coactivator in normal ZnS phosphors with green and blue emission.The solubility of these activators in ZnS is dependent on the presence of charge-compensating ions which usually function as coactivators (3). For instance, in ZnS: Cu with no chemical coactivator added, Froelich reported that only about 4x 104 g-atoms Cu/mole ZnS is retained in the lattice after firing (4). In contrast, at least 9x 10 -~ g-atom Cu/mole ZnS is retained in the ZnS:Cu,A1 phosphor with orange emission (5). Incorporation of the larger concentration of Cu in the latter case is possible because of the simultaneous incorporation of A1 as AI~S~. In this phosphor, the A1 was added to ZnS as an oxy-salt and converted to AI~S~ during the firing process in H2S at 1100~176It is questionable whether a large amount of AI~O~ would be converted completely to ALS~ when fired in the presence of ZnS in an H~S stream. Equimolar mixtures of ZnS and AI~O~, for instance, when so fired do not yield ZnALS,, the compound expected on complete conversion of the oxide to the sulfide. In all probability, addition of A1 as the oxide or oxysalt limits the solubility of Cu because of incomplete conversion to AI~S~. However, if the activator and coactivator are added as the ternary sulfide, the oxide conversion is circumvented. Further, activator and coactivator are added in exactly stoichiometric amounts in a form which facilitates incorporation in the lattice.It is the purpose of this paper to report results of studies in the systems CuGaS~-ZnS and AgGaS2-ZnS. Of special interest will be the structural data, limits of solubility, and luminescent properties. In these systems the concentrations of Cu or Ag in ZnS far exceed those reported in the literature, and with the increased concentrations of activators incorporated ...