Chalcogen (S, Se) doping has been explored as a route to further enhance hole conductivity both computationally and experimentally. The formation energies of Cu x S y and Cu x Se y competing phases within the chemical potential limits of CuI control the chemical potentials of the dopant atoms, but it is observed that synthesis conditions have little effect on the formation energies of the substitutional species S I and Se I due to the concomitant changing chemical potential of iodine. Much larger changes in formation energy are observed for the interstitial defects S int and Se int , where the changing chemical potential of iodine has no impact on the formation energy of the chalcogen defect.