The reaction of O(P-3) with HCl . M (M=HCl, Ar) complexes has been studied. While the monomer HCl, in its ground vibrational state, reacts extremely slow with O(P-3), it is shown here that the van der Waals complexes react with an efficiency of about 3 orders of magnitude larger than that of the monomer. The reactivity of DCl, on the other hand, is not enhanced by the complex formation. Molecular dynamics simulation indicates that the collision complex lifetime increases by several orders of magnitude due to the existence of the "third body" in the cluster. A model for explaining the complex induced enhancement of reactivity is presented and is supported by ab initio calculations.