Simultaneous profiles of methane, carbon monoxide, and ozone were measured using a balloon-borne remote sensor in August 1985. The instrument, which uses pressure-modulated radiometers, flew over the Canadian prairies and collected data for night, sunrise, early and late morning soundings in the altitude range 20--45 km. The data processing is discussed starting from the raw radiances and finishing with the retrieval of the concentration profiles. The programs used for calculating the theoretical radiances and the retrieval process are described in detail. The factors which need to be considered in the design of future experiments, particularly with regard to the filling pressure of the modulator cells are also addressed. The methane profiles show volume mixing ratios between ≈0.7¿10-6 v/v at 20 km and ≈0.8¿10-6 at 45 km. The ozone profiles show mixing ratios between ≈5¿10-6 v/v at 20 km and ≈7¿10-6 at 45 km and the carbon monoxide profiles between ≈0.5¿10-8 v/v at 20 km and ≈2¿10-8 at 45 km. These profile results are compared to data sets from other authors, and the similarities and differences are briefly discussed. ¿American Geophysical Union 1991