We present experimental results which clearly separate the various physical mechanisms which cause persistent photoconductivity in GaAs/AlxGa1−xAs heterojunctions. For high Al mole fraction the major contribution is from the donor‐relatedDXcenter. This contribution is eliminated by reducing the Al mole fractionx, but we observe a ‘‘residual’’ effect forx≲0.2. We show that this is due to the persistent photovoltage developed between channel and semi‐insulating substrate. Charge trapping in the epitaxial GaAs buffer layer contributes negligibly, contrary to the assumptions of other workers. This is demonstrated by fabricating modulation‐doped field‐effect transistors of low Al mole fraction on conductive substrates. In these devices persistent photoconductivity is eliminated as long as the substrate (back gate) potential is fixed with respect to the channel.