We report pseudodielectric function ⟨&egr;⟩ data for AlxGa1−xAs alloys of target compositionsx=0.00–0.80 in steps of 0.10 grown by liquid‐phase epitaxy and measured by spectroellipsometry. Cleaning procedures that produce abrupt interfaces between the technologically relevant alloysx≤0.5 and the ambient are described. The ⟨&egr;2⟩ data are corrected near the fundamental direct absorption edge by a Kramers–Kronig analysis of the ⟨&egr;1⟩ data to circumvent a limitation of the rotating‐analyzer ellipsometric technique. The results and the associated pseudooptical functions ⟨n⟩, ⟨R⟩, and ⟨&agr;⟩ are listed in tabular form. Accurate values of theE0andE1threshold energies are determined from these spectra by Fourier methods. From these values, and from similar values for a GaAs‐capped AlAs sample grown by organometallic chemical vapor deposition, the dependencies of theE0andE1interband critical point energies on nominal composition are obtained. Cubic polynomial representations of these dependences are determined to allow nominal Al fractions to be calculated analytically from optical threshold data. The systematic behavior of ⟨&egr;1⟩ at 1.5 eV and of theE2peak in ⟨&egr;2⟩ near 5 eV show that scatter in these data is less than 1% of the peak values of the spectra forx≤0.5. Forx≥0.6 the peak data appear to show systematic discrepancies indicating that chemical cleaning cannot completely remove surface overlayers on high‐Al‐content samples. Optical measurements for a sample withx=0.9 also reveal the oxidation of high‐Al samples proceeds irregularly and not along a uniform spatial front. Interpolation procedures to obtain approximate representations of dielectric function spectra at compositions other than those measured are discussed, and suggestions for improving accuracy in future optical measurements on these and related materials are also given.