Transport properties of Si‐doped Al0.35Ga0.65 As epitaxial layers grown by molecular beam epitaxy were studied and the activation energy of the Si donors determined. Previously reported values for the Si‐donor activation energy for x = 0.35 have varied considerably and values as high as 100 meV have been suggested. For x values above and below x = 0.35, the activation energies reported are much lower, and show better consistency. Anomalously high activation energies for Se (300 meV) and Te (150 meV) doped AlxGa1−x As near the direct‐indirect transition have also been reported. In this letter, a possible explanation for these anomalies in previous results based on the modulation doping effect is described. Results obtained for layers grown so as to avoid modulation doping indicate the activation energy of Si in Al0.35Ga0.65 As is less than 14 meV. When modulation doping effects were not suppressed, apparent activation energies as high as 125 meV were obtained. Finally, electron mobilities in samples not displaying modulation doping effects for an electron concentration of 6.5×1017 cm−3 were approximately 1 100 cm2/Vs.