We have used the quasiclassical trajectory method and a realistic potential energy surface to calculate rate constants and Arrhenius activation energies for dissociation of p-H2 in Ar from each of H2′s fifteen different vibrational levels at a rotational-vibrational temperature of 4500°K. We have also calculated the equilibrium rate constant and energy of activation and many other attributes of the equilibrium reaction at 4500°K. The effect of considering the seven quasibound states with longest unimolecular lifetimes as reactant states is also studied in detail. Reaction is favored by high internal energy-either high rotational quantum number at low vibrational quantum number or vice versa. In particular, at equilibrium the groups of vibrational levels with vibrational quantum numbers ν equal to 0-6, 7-11, and 12-14 contribute 26%, 48%, and 26% to the reaction rate. Dissociation from low ν levels proceeds primarily from the topmost j state for a given ν level; as a consequence neither the assumption of vibrational equilibrium nor the assumption of rotational equilibrium is valid for treating nonequilibrium effects.