The basic approach to determining composition of solid solutions combines X-ray spectroscopy and diffraction. Although careful preparation and measurements of the standards is required, the actual determination of composition of the unknown solid solutions is straightforward and rapid. Much flexibility is possible. For example, a plot of 20 (the diffraction angle) versus composition can be made which will speed matters further, with some loss in precision. Generally, the method described here is good to approximately 1 part in 10⁴, but with some additional effort an improvement of an order of magnitude can be achieved.