The dynamics of the O(P-3)+HCl and Ar-HCl reactions is investigated using a multiple configuration quantum/classical approach. In this work the dynamics of the hydrogen atom is propagated quantum mechanically in the three Cartesian coordinates of the atom, while the dynamics of the other atoms is propagated classically, in a center-of-mass frame. It is found that the introduction of the argon atom affects the reaction probability through two mechanisms. For nearly collinear O+Ar-HCl collisions, the argon atom blocks the transition state for the O+HCl reaction and inhibits the reaction. On the other hand when the collision geometry is such that the oxygen atom does not collide with the argon atom, the reaction probability is increased. These results are analyzed in terms of the shape of the ground state Ar-HCl wave function. The energy transfer dynamics from the oxygen atom and to the argon atom is also investigated. (C) 2003 American Institute of Physics.