The environmental effect of chlorofluorocarbon (CFC) replacement compounds such as the hydrochlorofluorocarbons (HCFCs) requires accurate data for gas phase absorption cross sections. Such data are particularly important in the near uv wavelength region where the cross sections are often very low. Measurements of such low cross-sections are difficult and unreliable. In the present work we have found that absorption cross sections derived from measurements in the liquid phase for two HCFCs, 1,1,1,2,2,- pentafluoro-3,3-dichloropropane (HCFC-225ca) and 1,1,2,2,3- pentafluoro-1,3-dichloropropane (HCFC-225cb) can, with appropriate correction factors, be converted into accurate gas phase values. The method itself has a basis in theoretical treatments which relate liquid to gas phase absorption cross section conversions. Thus a reliable experimental technique is available for determining gas phase absorption cross sections of diffuse absorbers in regions of very low absorptions. The method minimizes some of the systematic errors that accompany the measurement of extremely low gas phase absorbtivity in the ultraviolet and can potentially cover a wide range of temperature.