Three-dimensional fields of chemical and meteorological parameters over southwestern Germany were obtained by five aircraft during the TRACT experiment in September 1992. To ensure the homogeneous quality of the data and its compliance with the needs of data users, an extensive quality control and assessment (QA/QC) plan was devised and executed. The quality of O3, NO, NOy, SO2, and H2O2 measurements was assessed by intercomparisons at ground level against a set of standard instruments and in flight against each other. In-flight intercomparisons of air pressure, temperature, and humidity served also to assess the quality of meteorological measurements. The data quality objectives (DQOs) for TRACT were met with respect to all air quality parameters, except air temperature and humidity. The latter two measurements by different aircraft differed by a constant offset which can be corrected for.