Molecular arsenic, As4, is commonly used as the source for in situ p-type doping of Hg1−xCdxTe grown by molecular beam epitaxy. As incorporated, the arsenic is strongly self-compensated, requiring annealing for its p-type electrical activation. Here, a quasithermodynamic model is used to interpret the dependence of the arsenic concentration, cAs, as measured by secondary ion mass spectroscopy, on the incident As4 and Hg fluxes during growth. The results strongly suggest that the As4 is absorbed in its molecular form rather than being dissociated on the growth surface, as has previously been assumed. This clearly is relevant to the self-compensation of the arsenic in as-grown Hg1−xCdxTe. © 1999 American Institute of Physics.