A new mass spectrometer instrument based on the active chemical ionization technique was developed and flown to measure the stratospheric trace gases HNO3 and N2O5. In the active chemical ionization method primary ion species are injected into a flow tube with ambient stratospheric air. The primary ions are selected to produce characteristic secondary ions from ion-chemical reactions with the neutral gases to be detected. At the end of the flow tube, after a flow-time of about 30 ms, primary and secondary ions are analyzed with a double focusing mass spectrometer capable of measuring a full spectrum part simultaneously with the use of two electro-optical detectors. Profiles of HNO3 were determined using the conversion of chlorine cluster ions, generated by a discharge, into NO3 - clusters. Additionally the products from ion clustering reactions of CO3 - , produced by a photoelectron source, allowed an independent determination of a nitric acid density profile. In order to detect N2O5 (and ClONO2), negative iodine ions were used. Technical aspects of the new instrument and results from a balloon flight in November 1995 launched near León, in Spain at mid latitudes and obtained in an altitude range from 32 to 20 km will be presented.