A major challenge in modern organic geochemistry is the determination of the chemical composition of Earth’s major carbon reservoirs in marine and terrestrial systems. These reservoirs exchange carbon with the atmosphere and can vary between a source or sink for carbon dioxide (CO2) in a changing climate. Modeling rates of exchange between these reservoirs requires a comprehensive understanding of the relationships between chemical structure/composition and reactivity of compounds that comprise the organic carbon pool. Moreover, inter-reservoir conversion between refractory and labile carbon pools as a result of environmental factors and processing must be considered in these models. Identification of the compounds that are susceptible to conversion by photochemical and microbial processes is required for accurate estimates of CO2 flux between organic carbon reservoirs and the atmosphere. My research combines laboratory and field experiments in combination with modern analytical methods of analysis (e.g., excitation emission matrix fluorescence spectroscopy, ultrahigh resolution mass spectrometry, and nuclear magnetic resonance spectroscopy) to understand the structure and composition of organic compounds and the molecular-level transformations of these compounds from photochemical and microbial degradation processes.