Small‐area contrast fluctuations observed in cathodoluminescence‐mode SEM images of thin AlxGa1−xAs layers grown by liquid‐phase epitaxy on GaAs : Cr substrates are attributed to local variations in alloy composition. Quantitative estimates of the composition excursions are obtained from the variations in CL intensity by calibration against compositions known from electron‐probe microanalysis. In a typical sample, the CL variations are shown to correspond to peak‐to‐peak fluctuations of about 1 at.&percent; of Al and occur over irregular regions generally in the range 6–20 &mgr;m in diameter.