Lightly dopedp‐type GaAs films grown by molecular beam epitaxy on Cr‐doped substrates have been chemically analyzed using dynamic secondary ion mass spectrometry. Residual levels of Cr are, in general, below 4×1013cm−3. A major source of impurities in such material is identified. The effect of substrate temperataure on the concentration of B, Mg, Al, Si, Ca, Cr, Mn, Fe, and Cu are reported, and their likely effect upon the minority carier properties of the layers is discussed.