A series of CuGaS2crystals were annealed in excess sulfur at temperatures between 250 and 850°C. This treatment varies the room‐temperature hole density from 5 × 1015cm−3to 2 × 1018cm−3. From the temperature dependence of the resistivity, the acceptor ionization energies of several representative samples were determined. The energy increases from 5 to 70 meV as a function of increasing resistivity.