Extrinsic p-doped mercury cadmium telluride (MCT) layers have been grown by organometallic vapor phase epitaxy, using arsine in hydrogen as the dopant gas. Controllable doping in the range 3.5×1015–4.3×1016 was obtained when grown on GaAs with CdTe buffer layers. Consistently higher doping concentration (a factor of 2 to 4) was observed when a CdTe substrate was used. This is believed to be due to higher dislocation densities present when grown on GaAs, around which As may segregate. The cadmium fraction fell at very high flow; we believe that this is due to the prereaction of dimethylcadmium with arsine. Isothermal annealing under a Hg-rich ambient of MCT grown on CdTe substrates did not produce significant changes in the measured doping concentration. This indicates that the acceptor level is extrinsic in nature and that arsenic behaves as a stable acceptor dopant in MCT. The activation energy of this acceptor was determined as a function of doping, and is about one-half the value of the acceptor due to the Hg vacancies.