Perchloroethylene (PCE), a solvent used in dry cleaning, has been suspected of contributing significantly to photochemical ozone/oxidant (O3/O(x)) problems in urban atmospheres. Past evidence, however, was neither complete nor consistent. To interpret more conclusively the past evidence, and further understand PCE's role in the O3/O(x) problem, a smog chamber testing program was conducted. The program's objectives were: (a) to explain the mechanism of the PCE reaction in smog chamber atmospheres, and (b) to extrapolate the smog chamber findings regarding PCE reactivity to the real atmosphere. Results showed that (a) in smog chambers, PCE reacts and forms O3/O(x) following a C1-instigated photooxidation mechanisms rather than the OH-instigated mechanizm accepted in current smog chemistry, and (b) in the real atmosphere neither the C1-instigated nor the OH-instigated photooxidations of PCE can generate substantial concentrations of O3/O(x). In fact, PCE contributes less to the ambient O3/O(x) problem than equal concentrations of ethane.