The results of molecular dynamics calculations of thermal rates of diffusion of C and H atoms in Ar matrices are presented. The computed results are compared with those computed by a simple transition-state theory (STST). The results of the STST, which involve trivial computations, are in good agreement with the results of the classical molecular dynamics calculations, which require large amounts of computer time. These results show that the STST is adequate for predicting thermal rates of diffusion. The applicability of STST to diffusion in matrices provides an easy method for predicting rates since it greatly reduces the amount of information needed about the potential and effort in computation compared with methods such as Monte Carlo variational transition-state theory and molecular dynamics calculations. Calculations using STST were also carried out for O- and H-atom diffusion in xenon. The computed rates are smaller than the experimental values, indicating that the experimentally observed processes are likely occurring in matrices with imperfections, as has been suggested previously by others.