An atmospheric pressure ionization mass spectrometer (APIMS) was developed to determine atmospheric dimethyl sulfide (DMS) with a frequency high enough (25 Hz) to compute vertical fluxes in the marine boundary by eddy correlation. High precision and immunity to sample loss, fluctuations in instrument sensitivity, etc., were achieved by adding isotopically labeled DMS (d3 or d6) continuously to the manifold as an internal standard. Because mixing ratios are determined and not mass concentration, the vertical fluxes obtained did not suffer from Webb effect errors. High sensitivity at high humidities was achieved by efficient drying and by preheating the sampled air to about 400 ¿C. During the National Science Foundation (NSF) Dynamics and Chemistry of Marine Stratocumulus Phase II: Entrainment Studies (DYCOMS II) and Passing Efficiency of the Low Turbulence Inlet (PELTI) programs, DMS and d3-DMS (or d6-DMS) were each determined with integration times of 20 ms. From these data the ambient DMS level was computed every 40 ms. Vertical fluxes were determined with a relative precision better than 13%.