A photolytic flow system was used to study the primary and secondary steps in the reaction of O( 1D) with chlorofluorocarbons. Concentrations of reactants and products were determined by quadrupole mass spectrometry; varying contact times were obtained by altering the distance between the photolysis lamp and the sampling orifice. “Intermediate” quantum yields for ozone loss and for ClO production were then analysed to provide kinetic information. Preliminary results with CF 2Cl 2 as the halocarbon suggest that O( 1D), produced by ozone (O 3) photolysis, reacts to yield mainly CF 2Cl and ClO fragments. The CF 2Cl radicals then react with O 3, with a rate constant of the order of 10 −14 molecule −1 cm 3s −1 to produce a further ClO radical, probably together with CF 2. CF 2O appears only at relatively long contact times, suggesting that is not formed directly in a branch of the O( 1D) + CF 2Cl 2 reaction but rather in a secondary reaction between CF 2 and O 3.