Multiphoton dissociation of CF3C1 and CF3Br induced by CO2 laser pulses has been studied in the (0.1 - 60 torr) pressure range and (S - 30 Joule/cm2) average energy density range. The laser was adjusted on 1071.9 cm-1 and 1052.2 cm-1, resp., so as to excite preferentially the 13C isotopic molecules. The effects of pressure and energy on the isotopic selectivity were measured and a remarkable difference was found between the two molecules. As for the dissociation probabilities, they vary exponentially with the inverse of the energy. The applicability of such an Arrhenius-type relation is discussed and semi-quantitatively justified.