The first quantitative chemical analysis of polar stratospheric cloud particles has recently been performed using a balloon-borne aerosol composition mass spectrometer (ACMS). A similar spectrometer is presently used in a large cryo-chamber experiment to study low temperature aerosols. All experiments require prior to their employment an accurate calibration to convert mass spectrometer signals into molecular species contained in the aerosols. For the calibration, pure H2SO4/H2O droplets are generated having known composition and diameters between 0.4 m and 1 m. The size distribution and the number concentration can be controlled. A flow reactor with a rotating inner glass cylinder placed in a H2SO4/H2O bath solution of known concentration is used to condition the droplets. The residence time of the particles in the flow reactor is long enough that the droplets adopt the composition of the bath solution before entering the ACMS. The result is a linear relationship between the mole ratio of the H2SO4/H2O droplets and the mass spectrometer count rate ratio of water to sulfuric acid. The evaluation takes the dissociation of H2SO4 inside the ACMS into account. The calibration error varies between 3 and 4 wt. % H2SO4 for stratospheric particles with a composition of 30-70 wt. % H2SO4. Besides the calibration of the instrument, the analysis of the aerosols is a valuable diagnostic tool to investigate impurities in the particles.