Laboratory and atmospheric measurements of the isotopic composition of ozone have shown large and unusual 'non-mass-dependent' enrichments in O-17 and O-18 of similar to 10% relative to the O-2 from which it is formed. We present measurements of the bath gas and pressure dependence of the isotope enrichments in ozone formed by photolysis of O-2 in the presence of Ar, O-2, CO2, and SF6 from 50 to 760 Torr. The results provide new insights into the origin of the dynamically-driven symmetry-related isotope effect involved and new benchmarks for theory, including elimination of all isotope selectivity for ozone formation in SF6 by 700 Torr. (C) 2012 Elsevier B. V. All rights reserved.