The competitive adsorption of HNO3 and H2O from the gas phase onto fused silica surfaces is investigated. Brewster angle cavity ring-down spectroscopy is used to measure absorption of a laser probe beam by the HNO3/H2O coadsorbed on fused silica surfaces as a function of the mixture pressure. The laser absorption measurements were made in the 295-345 nm region. Langmuir adsorption constants for nitric acid and water were found to be 107 +/- 17 and 562 +/- 21 Torr(-1), respectively. A method has been developed for calculating absorption by HNO3 and H2O codeposited on the surface as a function of the HNO3/H2O mixture pressure using multicomponent Langmuir adsorption isotherms and absorption cross-sections at a given wavelength for surface-adsorbed HNO3 and H2O. The validity of this treatment has been evaluated both as a function of wavelength and as a function of mixing ratio.