The isotope fractionation of all ozone isotopologs of mass 48-54 u has been measured with high accuracy. The data represent a standard for the magnitude of the ozone isotope effect since pressure, temperature, and composition of the gas in which ozone is produced are well known. While (OOO)-O-17-O-17-O-17 and (OOO)-O-18-O-18-O-18 are depleted by -1.5% and -3.9%, respectively, all others are enriched with (OOO)-O-16-O-17-O-18 showing the highest value of 19.8%. Enrichments or depletions are independent of the isotope composition of the oxygen gas.