A study of common clinopyroxenes is presented including 40 new chemical analyses. The optical properties of these pyroxenes have been systematically determined with somewhat better accuracy than normally attained. Techniques for determination of these properties are discussed and their probable accuracy estimated. The results of the study are graphically presented on a series of curves. The recalculation of the chemical analyses in terms of a theoretical pyroxene formula and the distribution of various ions in the pyroxene structure are discussed.