The influence of tropospheric clouds on zenith sky light (or brief ZSL-DOAS) measurements of stratospheric gases is investigated. From a large set of intercomparison studies including six simultaneously operated UV/visible spectrometers, the zenith sky absorptions of O3, O4, NO2, and H2O are found to increase considerably under the investigated Cumulonimbus (Cb) cloud. The accuracy of the inferred visible O3 absorptions, however, are affected by interfering H2O absorptions. The increased cloudy sky absorptions are attributed to increased pathlengths due to multiple Mie scattering and hence interstitial gaseous absorptions inside the cloud. The absorptions detected for chemically inert gases like O4 (and H2O) are found to be inconsistent with those detected for NO2 and O3. This finding indicates that O3 and NO2 are modified by cloud related transport or chemical processes. ¿ 1999 American Geophysical Union