The oxidation of hydrocarbons affects nearly every living organism on the planet. Two of the most prevalent examples of this process are the combustion of hydrocarbon fuels and oxidative degradation of hydrocarbons in the atmosphere. The study of combustion and atmospheric chemistry is necessary as these are two of the most ubiquitous processes known and they can affect everything ranging from the economy to our health. In order to fully understand these very complex processes the reactive intermediates that are formed in them need to be investigated. Isolating and studying reactive intermediates can help provide better understanding of complicated reaction mechanisms and the elementary reactions that make up the mechanism. Positive spectral identification of reactive intermediates can help one gain greater insight into their structural properties, reaction kinetics, and overall chemical behavior. However, observing and identifying these species is not trivial. Due to their inherently reactive nature they are very short lived, exist in very small concentrations, and require spectroscopic techniques with high sensitivity in order to observe them. Once they are observed, high level quantum mechanical calculations need to be performed in order to analyze and assign their spectra. Two classes of reactive intermediates, peroxy radicals and Criegee intermediates, were the focus of this research and were observed in the near infrared (NIR) using moderate resolution cavity ringdown spectroscopy (CRDS). The Ã-X˜ trsansition of peroxy radicals is found in the NIR and was used to identify several peroxy radicals: C 6 -C 10 straight chain peroxy radicals (hexyl-decyl peroxy), one branched isomer of octyl peroxy (iso-octyl peroxy), an OH substituted peroxy radicals in 2,1-hydroxypropyl peroxy (2,1-HPP), snd two singly halogenated methyl peroxy radicals in chloromethyl peroxy (CH 2 ClO 2 ) and bromomethyl peroxy (CH 2 BrO 2 ). Spectral assignments for all but the C 6 -C 10 peroxy radicals have been aided by quantum mechanical calculations to determine origin band position and X˜ and à state vibrational frequencies; assignments for C 6 -C 10 peroxy radicals were determined by spectral/structural relationships. We also observed what we believe to be a transition from the 1 A' state to the low lying 3 A' state of the simplest Criegee intermediate, methylene peroxy (CH 2 O 2 ). While theoretical calculations are ongoing for this molecule to aid its spectral analysis we have developed an argument for the assignment of the carrier of the spectrum based on a strong analogy between the electronic structure of methylene peroxy and ozone, comparison with spectra collected for CH 2 ClO 2 and CH 2 BrO 2 , and the dependence of the spectrum on chemical conditions, specifically its reactivity with SO 2 .