CH4 and N2O measured by the balloon-borne Spectrom¿tre Infra Rouge pour l'¿tude de l'Atmosph¿re par Diodes Laser Embarqu¿es (SPIRALE) instrument at midlatitude on 2 October 2002 and in the polar region on 21 January 2003 are used to study the vertical structure of the stratosphere. SPIRALE utilizes a direct absorption technique using tunable diodes laser in the midinfrared. For the two flights, detailed structures with vertical extents less than 1 km and nonmonotonic profiles are observed. Measured N2O and CH4 have been compared with correlation curves deduced from ATMOS space shuttle measurements. The high vertical resolution (5 m) and high precision of the measurements allow us to discuss the air mass origin in detail. To help with the interpretation, the potential vorticity maps have been used. At midlatitude we have identified a tropospheric tropical air mass intrusion. The layer located just above has been identified as the "tropically controlled transition region." For this layer a specific correlation line has been deduced from our measurements. The nonmonotonic part of the profile in the 23--29 km altitude range corresponds to a transition layer where mixing is still occurring. For the polar flight the instrument has sampled air masses outside the polar vortex at the lower levels and inside the polar vortex above. An intense mesospheric descent is observed. We provide new CH4:N2O correlation values that can be encountered in cases of large mesospheric descent. Our measurements show also a nontypical CH4:N2O correlation line in a thin layer corresponding to a midlatitude filament crossing the balloon trajectory.