The kinetics of the hydroxyl radical (OH) + carbon monxide (CO) reaction, which is fundamental to both atmospheric and combustion chemistry, are complex because of the formation of the hydrocarboxyl radical (HOCO) intermediate. Despite extensive studies of this reaction, HOCO has not been observed under thermal reaction conditions. In this thesis, we report the development of a sensitive and multiplexed technique, time-resolved frequency comb spectroscopy (TrFCS) for the observation of reactive chemical intermediates on the microsecond timescale. Using this technique, we observed deuteroxyl radical (OD) + CO reaction kinetics and detected all relevant reactants and products. By simultaneously measuring the time-dependent concentrations of the OD, transDOCO, cis-DOCO and CO2, we observed unambiguous low-pressure termolecular dependence of the reaction rate coefficients for N2 and CO bath gases. These results confirm the HOCO formation mechanism and quantify its yield. Additionally, we observed cis-DOCO trans-DOCO isomerization and quantified its rate.