The oxygen isotopic composition (O-16, O-17, O-18) of the products produced in H-x, O-x reactions has been investigated using a fast flow reactor. The products were trapped at liquid nitrogen temperature. Measurement of all stable oxygen isotopes on reaction products enhanced distinction between mass-dependent and mass-independent fractionation processes. It was observed that all product species were mass-independently fractionated (MIF). Kinetic analysis suggests that MIF may derive from the H + O-2 + M reaction. Kinetic analysis and discussion of the potential role of symmetry reactions are presented. The similarity of isotopic behavior between this reaction and ozone formation suggests a common origin. Currently, no theory explains the mass-independent fractionation observed in an);gas-phase chemical reaction. Due to its atmospheric importance, possible isotopic implications for the Earth's atmosphere are discussed.