Oxygen diffusion mechanisms during oxidation of ZrB2-30 vol% SiC were explored at temperatures of 1500 degrees C and 1650 degrees C using an O-18 tracer technique. Double oxidation experiments in O-16(2) and O-18(2) were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy-dispersive spectroscopy, and time-of-flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO2 oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen-borosilicate surface at both 1500 degrees C and 1650 degrees C, while evidence of oxygen permeation was only observed at 1650 degrees C for short time (<1min) exposures. At longer times, >5-9min, complete oxygen exchange throughout both the borosilicate glass and ZrO2 was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.