A description of the phase diagram for Ga-S and structural studies of all the solid phases are given. Two types of compounds are found (1). " (\mathrm{Ga}{2} \mathrm{~S}{3}) " occurs in four forms. The monoclinic form (\alpha) ' is exactly stoichiometric and stable from room temperature to melting. The hexagonal form (\alpha) and the wurtzite-type form (\beta) exist only at high temperature; their formation requires a very small defect of sulfur. Passage from the hexagonal form (\alpha) to the wurtzite form (\beta) is progressive. The (\gamma)-blende-type form is clearly substoichiometric and only exists in a small domain of temperature. Gallium atoms are in tetrahedra for all the crystallographic forms. (2) Two varieties of (\mathrm{GaS}) are found: (2 \mathrm{H}) and (3 \mathrm{R}). A structural study is made using powder data for (\mathrm{GaS} 3 \mathrm{R}); this form is metastable for all temperatures of the solid state. Pairs of Ga are inside antiprisms of sulfur atoms for the (3 \mathrm{R}) form and inside triangular prisms for the (2 \mathrm{H}) form. Two liquid-liquid immiscible phases are observed between (\mathrm{Ga}) and (\mathrm{GaS}) and between (\mathrm{Ga}{2} \mathrm{~S}{3}) and (\mathrm{S}). 1993 Academic Press. Inc.