HArF and HKrF are chemically bound rare-gas compounds that have been produced by photolysis of HF and subsequent thermal annealing in the respective rare-gas matrices. In this paper we present a computational study of the delayed, thermally induced formation of these molecules in the matrix. Using realistic potentials for the molecular and guest-host interactions, the potential energy along the minimum energy paths for formation is evaluated, and thermal transition rates are computed using a Monte Carlo transition state method. A closely packed, dissociated configuration of the molecular fragments is found to play an important role, both as the possible trapping site of the photolyzed fragments, and as an intermediate structure for diffusion-controlled formation. The computed threshold temperatures of formation for the HArF and HKrF molecules at different matrix sites are in good agreement with experimental findings and with previous site assignments for these molecules. (C) 2003 American Institute of Physics.