The Absorbing Aerosol Index (AAI) was investigated and used to analyze GOME data and compare it to TOMS data. The physical interpretation of the AAI was studied with an extensive theoretical sensitivity analysis. The dependence of the method on a number of atmospheric, surface, and aerosol properties was studied using a numerical radiative transfer model. It was found to be sensitive to absorbing aerosols with wavelength-dependent refractive indices and to elevated absorbing aerosols, both with wavelength-dependent and wavelength-independent (gray) refractive indices. It was found to be insensitive to clouds, while small size scattering aerosols yield negative values. AAIs were calculated from GOME data for the period July 1995 to December 2000 and compared to TOMS AAI data. In a part of this period, July 1995 to October 1996, no TOMS observations were available, and the GOME data can be used to supplement the TOMS data set. The GOME AAI corresponds very well with known absorbing aerosol events. It suffers from lower spatial resolution and less frequent temporal coverage as compared to TOMS, but is useful as an independent data source of global aerosol measurements.