Ground based zenith sky measurements of O3 and NO2 slant optical thickness (SOT) from a field campaign at the Observatoire de Haute Provence in June 1996 are reported. Differential optical absorption spectroscopy in the 450--497 nm region was used to derive SOT for both species under clear sky, cloudy and thunderstorm conditions. SOT enhancements of 62% for O3 and up to 320% for NO2 are found in a thunderstorm cloud. Interpretation of the measurements was carried out using a radiative transfer model. To explain the measurements a mean increase of 38 ppbv of the in-cloud concentration of O3 has to be assumed. This amount leads to a mean in-cloud concentration of more than 100 ppbv of O3. The corresponding increase in NO2 is 3 ppbv, resulting in an in-cloud concentration of 3.2 ppbv. The NO2 enhancements are in large part explained by lightning processes. In contrast, the enhanced O3 appears to be induced by intrusion of stratospheric air and possibly ozone production by non-lightning discharge mechanisms. ¿ 1999 American Geophysical Union