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Although the Al_xAs alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of . Here a new Al_xAs calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such “standard’’ Al_xAs layers were measured by high-resolution x-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The phenomenological equations derived from these measurements can now be used to establish the Al content of unknown alloys with confidence. In addition, the results show that Vegard’s law does not hold for the variation of the Al_xAs lattice constant with . The small quadratic term has very important implications for a correct analysis of x-ray results.