Society today is still very dependent on petroleum, the most complex naturally occurring mixture on the planet. Ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been employed to comprehensively characterize petroleum, because of its ability to resolve and identify hundreds of thousands of compounds in complex mixtures. However, despite advances in the chemical knowledge of fossil fuels, many questions remain unanswered regarding their behavior in the environment. Oil spills, such as Deepwater Horizon (DWH) are harmful to coastal and marine ecosystems, as well as public health, however, are inevitable in an oil-dependent world. Therefore, understanding the molecular composition of petroleum and how it transforms in the environment is crucial for the development of better clean-up methodologies. Specifically, photooxidation from sunlight has been indicated as a primary source of weathering of spilled fossil fuels. This process increases the oxygen content of hydrocarbons, which leads to an increase in its interaction with water. This can lead to the formation of emulsions, which were observed in the Gulf of Mexico after DWH and made clean-up more difficult, due to their resistance to dispersants. Extensive photooxidation of petroleum has been shown to cause dissolution in water; the production of water-soluble hydrocarbons can harm ecosystems due to their likely toxicity. Furthermore, asphaltenes, the alkane-insoluble fraction of crude oil, have high oxygen content before photooxidation, and therefore are especially important in the formation of emulsions and water-soluble species through photochemical reactions. Asphaltenes are comprised of highly aromatic structures which can undergo photooxidation and photofragmentation to produce abundant water-soluble compounds; moreover, these compounds make up a large portion of materials extensively used in the built environment, such as road asphalt and roofing tiles. Chapter 1 provides an introduction to FT-ICR MS and explains why it is crucial for analysis of petroleum. Chapter 2 discusses the identification of ketones as photooxidation products from petroleum in laboratory studies and DWH field samples; aromatic ketones are of interest due to their demonstrated toxicity in animals. In Chapter 3, the effect of sulfur-containing compounds on photochemistry of light and heavy crude oils is compared; sulfur species (e.g. sulfides) are known for their toxicity and are thus important to understanding environmental impacts in spills. Chapter 4 expands studies to determine the role of molecular structure in the formation of oil- and water-soluble photoproducts through the isolation and solar irradiation of compounds with varying numbers of aromatic rings. Chapter 5 demonstrates how advanced ion isolation methods coupled with MS/MS can provide structural information about asphaltenes from petroleum and coal, which is central to understanding the role of molecular structure in the production of water-soluble compounds. Thus, Chapter 6 covers the effect of asphaltene structure on the molecular composition of the photooxidation products. Asphaltene samples enriched with different structural motifs, island vs. archipelago, were photoirradiated. The results demonstrate that only archipelago structures contribute to the significant production of water-soluble compounds. The leachable photoproducts formed through solar irradiation of road asphalt are described in Chapter 7. Finally, it is important to highlight that petroleum photooxidation increases the concentration of interfacial material (IM). IM consists of highly oxygenated compounds that interact strongly with water and therefore contribute to emulsion formation / stability in the environment as well as refinery processes. Chapter 8 identifies emulsion-promoting compounds and investigates improved methodologies for the isolation of these species from petroleum and field samples.