Photodissocn. of H2O (D2O) and Cl2 trapped in rare gas matrixes was studied in relation to the cage effect. Barrier energies for the exit of the fragments from the matrix cage were detd., together with the excess kinetic energy dependence of the photodissocn. quantum efficiency. Simple anal. models and classical mol. dynamics simulations are used to discuss the results. [on SciFinder (R)]