A new empirical technique for the fast retrieval of vertical concentrations of water vapor from the GOME uncalibrated spectra is described in detail. The ERA technique (Empirical Retrieval of Atmospheric air mass factor) makes maximum use of GOME's capability to measure atmospheric spectra over a broad wavelength range with high spectral resolution, by retrieving a normalization factor for the water vapor concentrations directly from the data (oxygen slant column measurements). ERA results are compared to the ECMWF TCWV analysis (Total Column Water Vapor), showing good agreement. ¿ 2000 American Geophysical Union