During the 2002 environmental satellite Envisat mid-latitude validation campaign, a new upgraded configuration of the Infrared Balloon Experiment (IBEX) Fourier transform spectrometer, which had its first flight in 1978, performed a stratospheric balloon flight across the Mediterranean Sea. Among the substantial upgrades made to the instrument, the use of photon-noise-limited detectors permitted us to reach the theoretical limits in terms of signal-to-noise ratio. Also, important modifications were made to the interferometric system and electronics, such as the installation of a solid-state laser reference source and an onboard data recording system. During the flight, measurement of volume-mixing-ratio vertical profiles of {\mathrm{O}}_{\mathrm{3}} , {\mathrm{HNO}}_3 , {\mathrm{N}}_{\mathrm{2}}\mathrm{O} , and \mathrm{ClO} from an altitude of \approx 38\mathrm{km} were performed with a vertical resolution of \approx 1.5\mathrm{km} .