The oxidation of thin films of boron and hafnium diboride grown on a Hf(0001) single crystal surface was studied with reflection absorption infrared spectroscopy (RAIRS). For both the boron and hafnium diboride films an intense broad band near 1500 cm(-1) as well as features near 750 and 1250 cm(-1) were detected. These vibrational features are characteristic of solid B2O3. The formation of B2O3 is also indicated by x-ray photoelectron spectroscopy (XPS) by the appearance of a chemically shifted B(1s) peak at a binding energy of 193.1 eV. However, the sensitivity for detecting surface oxide formation is considerably higher for RAIRS than for XPS.