The accurate measurement of the gaseous sulfuric acid concentration is crucial within many fields of atmospheric science. Instruments utilizing chemical ionization mass spectrometry (CIMS) measuring H2SO4, therefore, require a careful calibration. We have set up a calibration source that can provide a stable and adjustable concentration of H2SO4. The calibration system initiates the production of sulfuric acid through the oxidation of SO2 by OH. The hydroxyl radical is produced by UV photolysis of water vapor. A numerical model calculates the H2SO4 concentration provided at the outlet of the calibration source. From comparison of this concentration and the signals measured by CIMS, a calibration factor is derived. This factor is evaluated to be 1.1 x 10(10) cm(-3), which is in good agreement with values found in the literature for other CIMS instruments measuring H2SO4. The calibration system is described in detail and the results are discussed. Because the setup is external to the CIMS instrument, it offers the possibility for future CIMS intercomparison measurements by providing defined and stable concentrations of sulfuric acid.