We report a new experimental method, reaction modulation spectroscopy, (RMS), that shows great promise in enabling the systematic analysis of complicated oxidation mechanisms over the full range of atmospheric pressures and temperatures. The method is a form of difference spectroscopy in which we employ FTIR absorption spectroscopy in a high-pressure flow system (HPFS) where a plume of reacting species is examined before any significant fraction reaches the tube wall. The onset of the reaction is modulated by modulating the flow of an initiating radical species over a period that is short compared to any associated with experimental drifts. Spectra obtained with the radical source on and off are ratioed, giving a transmittance spectrum showing only the effects of the reaction modulation. The system is in a steady state, so signal averaging over long periods (up to several days) may be employed, if necessary, to obtain a high signal-to-noise ratio. Because no bulk reagents are disturbed in the process, we observe extremely precise conservation of mass in the reaction plume. We illustrate the technique for the system OH + C3F6 (+ O-2, NOx,...) --> --> CF2O + CF3CFO where we observe nearly 100% conservation of odd-nitrogen species and roughly 90% conservation of carbon under conditions chosen to force the reaction to completion, with residual spectra consistent with unidentified minor products having cross sections similar to the observed aldehydes.