Calculations have been made of the barrier heights and recombination velocities at semiconductor grain boundaries subject to uniform illumination with above‐band‐gap light. A detailed balance approach has been coupled with a solution of the current continuity equation to yield a full solution of the problem subject to the approximation that majority carrier currents can be properly described by thermionic emission expressions. Silicon bicrystal data are presented and shown to be in good agreement with the predictions of the theory.