The aerosol optical thickness (AOT) is a widely used parameter to study aerosol effects on the Earth's climate. AOT is commonly derived from direct solar irradiance measurements with a sunphotometer. This type of instrument is best calibrated at a high-altitude site. In Europe, the High Altitude Research Station Jungfraujoch (3580 m a.s.l., Switzerland) offers excellent facilities to perform sunphotometer calibrations. Between August and November 2003 we used several instruments at this high altitude station to perform Langley-based calibrations and AOT determinations. Good agreement between the AOTs of all instruments was obtained even though different algorithms and extinction models were used. The poster shows results for the calibrations and the AOTs as determined at the Jungfraujoch during the calibration campaign. To reveal the effect of the different retrieval methods on the AOTs in more detail we will show how the results of all instruments compare to each other when exchanging the algorithms and extinction models between the instruments.