Summary: An X-ray investigation has been made of synthetic pyroxenes in the system acmite-diopside, crystallized at 750° C and 1000 kg/cm² water-vapour pressure. Lattice parameters of these pyroxenes have been determined by least squares analyses of powder diffraction patterns and show an approximately linear relationship with composition, plotted as weight per cent. A rapid X-ray method has also been devised for the determination of the compositions of pyroxenes of this series.