An atmospheric pressure ionization mass spectrometer (APIMS) was developed to determine atmospheric sulfur dioxide (SO2). High precision and immunity to sample loss, fluctuations in instrument sensitivity, etc., were achieved by adding isotopically labeled SO2 (34S16O2) continuously to the manifold as an internal standard. During the NASA Transport And Chemical Evolution Over The Pacific (TRACE P) program and the National Science Foundation (NSF) Passing Efficiency of the Low Turbulence Inlet (PELTI) program, 32S16O2 and 34S16O2 were each determined with integration times of 20 ms. From these data the ambient SO2 level was computed every 40 ms. In the PELTI studies at SO2 levels of approximately 60 parts per trillion by volume (pptv), vertical fluxes were determined with a precision better than 10%. When the data were averaged to 1 s, the lower limit of detection was <1 pptv at the 2σ level. Isotopomer ratios observed during the Aerosol Characterization Experiment-Asia (ACE Asia), intercomparisons of APIMS and gas chromatography/mass spectrometry with isotopically labeled internal standard on the same aircraft in test flights and PELTI, and the intercomparison of two APIMS instruments on different aircraft during ACE Asia and TRACE P, are presented to support the high sensitivity, accuracy, specificity, and high time response of the instrument.