Quantum mechanical rate constants are computed for the collinear reaction H + Cl2 →HC1 + Cl using the reaction probabilities of Baer. For comparison we also computed reaction probabilities and rate constants for this reaction using (a) the quasiclassical trajectory method, (b) the reverse quasiclassical trajectory method, (c) the classical S matrix theory (using realvalued trajectories only), and (d) transition-state theory assuming separability of the reaction coordinate at the transition state. Comparisons are made not only for total reaction probability and total rate constant but also in general for state-to-state reaction probabilities and state-to-state rate constants. The quasiclassical trajectory method is generally accurate except in the threshold regions for various state-to-state processes. It is more accurate for total reaction probabilities and total rate constants than for state-to-state reaction probabilities and rate constants. The quasiclassical trajectory calculations of total rate constants for reaction in a given initial vibrational state agree with the quantum calculations within 29% for the 300–1000 K temperature range but the state-to-state rate constants may be in error by a factor of 2 or more even for processes which are classically allowed in the sense of classical S matrix theory. Classical S matrix theory does not always provide a more accurate way to extract state-to-state reaction probabilities from these trajectories. Transition state theory (which yields average reaction probabilities and total rate constants for a thermal distribution of initial states but does not yield state-to-state results) is fairly accurate for this reaction even with the assumption that the reaction coordinate is separable.