The photochemistry of stratospheric ozone in the tropics was intensively investigated during a balloon campaign at Teresina/Brazil in June 2005. Our limb scanning UV/Vis mini-DOAS instrument was deployed on 3 balloon flights on June 14 (MIPAS-B), June 17 (LPMA/DOAS) and June 30 (LPMA/IASI). During one of this flights (MIPAS-B), we could surprisingly detect HONO in the tropical tropopause transition layer (TTL). Observed HONO profiles were almost time independent for three hours around local noon with maximum HONO concentrations of 1.0x 10^9 molec/cm3 at 11 km. The measured UV/vis skylight spectra were analyzed applying the Differential Optical Absorption Spectroscopy (DOAS) method. When combined with 3D radiative-transfer modeling and an optimal estimation inversion technique, atmospheric concentration profiles of the targeted trace-gases can be inferred for each limb scan [Weidner et al., 2005]. Meteorological data and simultaneously measured O3 and NO2 indicate that the measured HONO can be best explained by intense radical photochemistry in the outflow of a meso-scale convective system, which possibly includes lightning produced NOX and HOX and, subsequent, HONO production. However, in order to explain the large amount of detected HONO by known chemical reactions, an elevated Leighton ratio and /or intense HOX photochemistry are necessary.