A study was made of the correlation between substrate flaws and the performance of GaAs injection lasers fabricated by liquid‐phase epitaxy. These devices arep+/nstructures where thep+ region is deposited epitaxially, followed by heat treatment to displace thep‐njunction about 1–2 &mgr; into then‐type substrate. The melt‐grown substrates were studied with the aid of transmission electron microscopy, infrared transmission, x‐ray topography, etching studies, and photoluminescence. It is concluded that the dislocation density of the substrate is not the controlling factor in laser performance unless it exceeds 105cm−2. A dislocation density of 103–104cm−2is not excessive. Precipitates and impurity striations in the substrate are probably the most damaging imperfections. In Te‐doped GaAs the presence of Ga2Te3precipitates is to be avoided. In the case of Si‐doped materials, the precipitation problem appears to be less severe. Nevertheless, small clusters have been detected by electron transmission microscopy in the back portion of some melt‐grown ingots (furthest from the seed end). Their nature is unknown but they severely degrade laser performance. We show that a good correlation exists between the relative photoluminescence efficiency and laser performance. Thus, photoluminescence measurements are useful in the selection of Si‐doped substrates.