Dissolved organic carbon (DOC) is a significant component in aquatic carbon budgets and its fate has implications for carbon biogeochemistry, and ultimately for ecosystem functioning. We have been examining DOC composition at the molecular-level in the Verde River and in Tempe Town Lake using liquid chromatography coupled to electrospray-ionization mass spectrometry (LC ESI-MS). This exciting technique examines the extremely complex mixture of polar organic compounds in water provides both a broad picture or ‘fingerprint’ of the compounds present and specific-compound identification. This technique provides a new understanding of DOC sources and fate in the urban systems because it allows us to examine not only the amount of carbon present but also the composition of the carbon. This molecular-level analysis of the range of compound types that can be extracted from soils and transformed in aquatic will provide new insights that begin to unlock the ‘black box’ of DOC composition. The ESI-MS system generates very large datasets - individual water samples produce spectra with 1000’s of peaks. The assessment of both large- and small-scale difference among spectra is key to understanding the processes transforming carbon in the environment. This request for summer salary support will allow me to spend the time required to develop techniques for assessing these large and complex datasets. This work is relevant to Fluxes of Materials and Socio-Ecosystem Response IPA, the project-wide 200-Point Survey, as well as the general research area of Biogeochemical Processes.