Characterizing transient solution-phase intermediates of catalytic reactions at ambient conditions is a long-standing problem in analytical chemistry. Electrospray ionization mass spectrometry (ESI-MS) is well-established for studying reaction mechanisms in solution. Stopped- and continuous-flow ESI-MS can achieve time resolution in the millisecond (ms) regime (as low as ∼5-10 ms) but their complicated experimental configurations limit throughput and the scope of reactions available for study. In addition, these methods are prone to carryover effects, which can cause unwanted side reactions during analysis. Ambient mass spectrometry (MS) techniques such as reactive desorption electrospray ionization (rDESI) has resolved many of these problems, achieving reaction time resolution in the millisecond regime. Herein, we have successfully developed an ionization source based on transmission mode DESI (TM-DESI) called multi-stage reactive TM-DESI (rTMn-DESI, where n represents the number of desorption stages) that allows real-time capture of intermediates on submillisecond timescales. In addition, this technique allows control of the order in which reagents are introduced, providing the ability for step-wise elucidation of fast catalytic solution-phase processes and for minimizing unwanted degradation reactions. Using this technique, we characterize intermediates of various catalytic reactions such as C-H hydroxylation via high-valent iron oxo porphyrin complexes.