Compositions of arsenopyrite coexisting with pyrite from eight different mesothermal gold-bearing systems in the Otago Schist fall mainly into a narrow band of 30+ or -1 at. percent As. Intragrain and intergrain compositional variations, combined with analytical uncertainty, preclude resolution of differences in arsenopyrite compositions between samples of less than 1 at. percent As. Only one of the vein systems has an arsenopyrite compositional range that extends below 28 at. percent As. Arsenopyrite from a pyrrhotite-bearing assemblage has compositional range extending above 32 at. percent As. The vein systems have formed over a wide range of temperatures between about 200 degrees and 400 degrees C. It is not possible to resolve the influence of temperature on arsenopyrite compositions for most samples. Sulfur and oxygen activity of the hydrothermal fluid may be responsible for the low degree of arsenopyrite compositional variation.