The composition of the borosilicate glass layer formed during oxidation of ZrB2-30 vol% SiC was determined to elucidate the extent of B2O3 retention in the oxide during high-temperature oxidation. Oxidation was conducted in stagnant air at 1300 degrees C, 1400 degrees C, and 1500 degrees C for times between 100 and 221min. Specimens were characterized using mass change and scanning electron microscopy. After oxidation, the borosilicate glass layer was dissolved from the specimens sequentially with deionized H2O and HF acid, to analyze the glass composition using inductively coupled plasma optical emission spectrometry. It was found that the average B2O3 content in the glass scale ranged from 23 to 47mol%. Retained B2O3 content in the bulk of the glass decreased with increasing temperature, confirming increased volatility with temperature. Boron depth profiles were also obtained in the near surface region using X-ray photoelectron spectroscopy and energy dispersive spectroscopy. The measured B concentrations were used to estimate the B2O3 concentration profile and B diffusion coefficients in the borosilicate glass. Implications for the ZrB2-SiC oxidation process are discussed.