A three‐layered AlxGa1−xAs&sngbnd;GaAs&sngbnd;AlxGa1−xAs structure has been used to measure the optical absorption and photoluminescence in thin GaAs layers prepared by liquid‐phase epitaxy. The results presented here are for lightly dopedn‐type GaAs with free‐carrier concentrations near 1016cm−3; however, the technique can be used for arbitrarily doped material. The absorption coefficient &agr; was measured between 1.4 and 2.2 eV at 2 and 298 K. The absorption strength at the band gap was found to be (1.15×104)±1000 cm−1at 2 K and (0.99×104)±1000 cm−1at 298 K. At 1.96 eV, the energy of the He&sngbnd;Ne laser commonly used for photoexcitation of GaAs, &agr; at 298 K was measured to be 4.4×104cm−1. A value of 3.8 meV for the room‐temperature exciton binding energy was inferred from the temperature dependence of the interband absorption strength. This value together with previous reflectivity data for high‐purity GaAs gives the energy gap of pure unstrained GaAs at 298 K as 1.424±0.001 eV. The effects of strain due to lattice mismatch in the three‐layered structures were observed in the absorption edge at 2 K. The calculated photoluminescence spectrum obtained through the principle of detailed balance from the absorption data agrees well with the measured photoluminescence at 298 K. A comparison of the photoluminescence from the excited surface and back surface permits assignment of an upper limit of 5×104cm/sec for the room‐temperature GaAs&sngbnd;AlxGa1−xAs interface recombination velocity forx≈ 0.5. This comparison of the front and back photoluminescence also shows that the minority‐carrier diffusion length at 298 K for a sample with an electron concentration of 2×1016cm−3is at least 2.5 &mgr;. The data presented here can be used to calculate the intrinsic carrier concentrationni, the thermal radiative generation rateG, and the radiative constantB(=Rr/np). The values at 298 K (corrected where necessary to a band‐gap energyEg=1.424 eV) are as follows:ni=1.8×106cm−3,G=4500 cm−3/sec, andB=1.4×10−9cm3/sec.