We report the results of in situ doping of HgCdTe by molecular beam epitaxy (MBE) using an elemental arsenic source. Arsenic was successfully incorporated as an acceptor during the MBE growth using either a conventional or planar doping approach. Hall effect measurements and secondary ion mass spectroscopy (SIMS) were used to investigate arsenic activity before and after different Hg anneals. Close to 100% acceptor doping efficiency with arsenic was obtained on these MBE grown layers after annealing. This corresponds to a total arsenic concentration of approximately 2 X 10(18) cm(-3) which is more than sufficient for a wide range of infrared devices. At much higher total arsenic concentration, electrical activity falls off drastically.