Inductively coupled plasma optical emission spectroscopy is shown to confirm a recent correlation between photoluminescence (PL) peak energy for AlGaAs epitaxial films and the Al mole fraction x of those films. These two methods also agree within their expanded uncertainties with the Al composition as determined by growth rate measurements using reflection high energy electron diffraction intensity at the time of specimen growth. No systematic variations between the three methods as a function of Al mole fraction were observed. The lowest uncertainty was found in the PL measurements, allowing certification of Al mole fraction x in standard reference materials to an expanded uncertainty of 0.003 for x < 0.35. Details of the uncertainty analysis, as well as possible improvements in those uncertainties, are discussed.