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Using variable angle spectroscopic ellipsometry, optical constants for AlAs (1.4-5.0 eV) are presented which are simultaneously compatible with measured data from four different samples. The below-gap index values are compatible with published prism measured values. The second derivative spectrum are compatible with published values above the direct band gap. The AlAs spectra is Kramers-Kronig self-consistent over the measured range and is compatible with published values from 0.6 to 1.4 eV. The optical constants for thin (<50 >Å) GaAs caps on AlAs are shown to be different from bulk GaAs values and require special consideration when fitting ellipsometric data. For the thin GaAs caps, the E1 and E1+ Δ1 critical-point structure is shifted to higher energies as previously observed for GaAs quantum wells. Bulk AlAs optical constants are shown to be different from those of a thin (~20 Å) AlAs barrier layer embedded in GaAs. The thin barrier layer exhibits a highly broadened critical-point structure. This barrier broadening effect (AlAs) and the thin cap shifting effects (GaAs) have implications for in situ growth control schemes which make use of the E1 and E1+ Δ1, critical-point region.