The GOME instrument consists of a spectrometer and scan mechanism to provide spectral radiance measurements of the earth's atmosphere over the entire spectral range 240 to 790 nm. The photometric calibration of the instrument is accomplished with a separate calibration unit including both calibration lamps and a diffuser to direct solar radiation into the spectrometer. In this paper we concentrate on a validation activity using the solar irradiance as a well calibrated source of known illumination, and from the GOME instrument response we derive the sensitivity of the instrument and changes in the instrument response with time. As the "known" solar input we use daily observations of the SOLSTICE instrument on NASA's Upper Atmosphere Research Satellite (UARS). SOLSTICE covers the spectral range from 120 to 420 nm, and the overlap with GOME spectral range provides a calibration of channels 1 and 2. The 2σ absolute calibration of the SOLSTICE data is 4%. In addition, the 2σ relative accuracy of the SOLSTICE data set is approximately ±2%, and helps to determine trends and changes in the GOME instrument response. The GOME/SOLSTICE comparison establishes characteristics of the GOME instrument in the solar irradiance configuration only, and additional and ancillary information on the solar diffuser and scan mirrors is also required to establish the calibration of GOME for radiance observations.