Many important gas-phase processes (e.g. in Atmospheric and Combustion Chemistry) involve vast networks of coupled chemical reactions that occur simultaneously. Some of these reactions proceed via metastable intermediates, resulting in very complex T- and P-dependent behavior. The study of such systems often requires development of new experimental tools, capable of probing of numerous chemical species at once over a wide range of conditions. This seminar will describe the development of two new experimental techniques, time-resolved broadband cavity-enhanced absorption spectroscopy (TR-BB-CEAS) and high-pressure multiplexed photoionization spectrometry (HP-PIMS). I will also present some recent successes in identifying and characterizing the elusive short-lived chemical species that govern the fate of many gas-phase phenomena. I will focus on the reactions of Criegee intermediates, key players in the Earth’s troposphere, and on the radical chain-branching chemistry in low-temperature autoignition.