Hall effect and resistivity measurements on single crystal samples of ZnSe doped with various amounts of gallium were performed both at high temperature while the crystals were in equilibrium with atmospheres of known zinc pressures and after cooling to room temperature. At high temperatures, gallium present at a low concentration behaves as aluminum and acts as a single donor. At high concentrations and high zinc pressures, the donor efficiency is strongly reduced. This reduction is attributed to the precipitation of a fraction of the gallium as GaSe. Cooled crystals with low Ga concentrations show a similar behavior, the only difference being a sharp reduction in electron concentration at low pzn when [VZn′] ≥ [e′]. At high Ga concentrations and high zinc pressures, black precipitates are formed near the surface during cooling. In addition, changes take place in the ZnSe phase which strongly reduce the electron concentration. These changes are believed to consist of the transfer of Ga from Zn to Se sites with formation of centers acting as multiple acceptors and electron traps. © 1978, The Electrochemical Society, Inc. All rights reserved.