High quality ZnSe:Cl has been grown on GaAs by gas source molecular beam epitaxy (GSMBE); elemental Zn and H2Se are used as source materials, with ZnCl2 as a dopant source for donors. Atomic Cl concentrations ([Cl]) approaching 1020 cm−3 have been incorporated into the lattice as indicated by secondary ion mass spectrometry (SIMS). At incorporation levels greater than 1020 cm−3, an appreciable decrease in the growth rate has been observed. The sharp transition to a negligible growth rate is attributed to the occurrence of a surface chemical reaction originating from Cl and H which are present in the GSMBE environment. For [Cl] as high as 4 x 1018 cm−3, the films exhibited high crystalline quality, as indicated by photoluminescence originating from a single intense donor-bound excitonic transition. Hydrogenation of semiconductors can potentially result in the electrical passivation of incorporated acceptor and donor species. In the case of ZnSe:Cl, H was present in the ZnSe layers, but did not appear to adversely affect the electrical properties of the n-type films. In contrast, for the growth of ZnSe:N, where a nitrogen plasma cell was employed as a source of nitrogen, the H concentration (as determined by SIMS) was observed to track the N concentration. The ZnSe:N films were highly resistive for various amounts of N incorporation, which suggests that H incorporation is an issue of primary importance in the p-type doping of ZnSe grown by GSMBE.