Ozone was produced by UV photolysis (157--200 nm) of molecular oxygen. The product ozone is enriched in 17O,18O on a mass-independent basis, similar to the fractionations observed in an electrical discharge. The magnitude of 18O,17O enrichment decreases with decreasing pressure, suggesting the presence of two fractionation mechanisms with differing Δ17O/Δ18O ratios. One of the end members would be mass-independent with Δ18O≂Δ17O≂90%, which is dominant above ~20 torr. The second process which is significant below ~6 torr produces O3 with Δ18O≂-55%, Δ17O≂-27.5, possibly due to isotopic selectivity in diffusion to the reaction systems wall. The Δ17O=Δ18O isotopic fractionation resembles that observed in stratospheric ozone, though the magnitude of fractionation is a factor of four less. ¿American Geophysical Union 1987