In this letter we have investigated then‐type doping properties of Zn1−xMgxSe grown by molecular beam epitaxy using a solid ZnCl2source as dopant. We describe the effect of dopant source temperature on the carrier concentration and show that the optimum doping temperature changes with varying Mg compositions. At the same time, the highest achievable electron concentration decreases as the Mg content in the sample increases, and this behavior can be described by a model that assumes that the Fermi level is pinned 120 meV above the conduction band edge of ZnSe, in an absolute energy scale. ©1995 American Institute of Physics.