A photoluminescence technique was used to identify residual donors in the 1014 cm−3 range in high-purity GaAs grown by metalorganic chemical vapor deposition. The measurements were taken at zero magnetic field. A narrow-linewidth tunable laser was used to resonantly enhance a specific narrow-linewidth ‘‘two-electron’’ satellite line of the donor-bound exciton, enabling determination of the 1s-to-2s donor energy separations. Ge was identified as the dominant residual donor. Lower levels of the donors X1 and X2 were observed in some samples. These are attributed to Si (or possibly Sn) and S donors.