Photoluminescence time‐decay measurements on Ga0.5Al0.5As/GaAs double heterostructures were made over a wide range of GaAs active layer thickness and doping levels at room temperature. Observed decay times &tgr; in variously doped GaAs samples range from 10 to 450 nsec. Effects of self‐absorption of luminescence and doping level are demonstrated for GaAs layer thicknessd≳1 &mgr;m. Ford<1 &mgr;m, the observed decay times are nearly independent of doping level and vary almost linearly withd. The data are interpreted in terms of a small interfacial recombination velocity (Si=450±100 cm/sec) at the Ga0.5Al0.5As/GaAs interface. The value ofSidetermined here is an average over the doping levels examined.