This thesis presents the results of four discrete investigations into processes governing the organic and inorganic chemical composition of seafloor hydrothermal fluids in a variety of geologic settings. Though Chapters 2 through 5 of this thesis are disparate in focus, each represents a novel investigation aimed at furthering our understanding of subsurface geochemical processes affecting hydrothermal fluid compositions. Chapters 2 and 3 concern the abiotic (nonbiological) formation of organic compounds in high temperature vent fluids, a process which has direct implications for the emergence of life in early Earth settings and sustainment of present day microbial populations in hydrothermal environments. Chapter 2 represents an experimental investigation of methane (CH4) formation under hydrothermal conditions. The overall reduction of carbon dioxide (C02) to CH4, previously assumed to be kinetically inhibited in the absence of mineral catalysts, is shown to proceed on timescales pertinent to crustal residence times of hydrothermal fluids. In Chapter 3, the abundance of methanethiol (CH3SH), considered to be a crucial precursor for the emergence of primitive chemoautotrophic life, is characterized in vent fluids from ultramafic-, basalt- and sediment-hosted hydrothermal systems. Previous assumptions that CH3SH forms by reduction of CO2 are not supported by the observed distribution in natural systems. Chapter 4 investigates factors regulating the hydrogen isotope composition of hydrocarbons under hydrothermal conditions. Isotopic exchange between low molecular weight n-alkanes and water is shown to be facilitated by metastable equilibrium reactions between alkanes and their corresponding alkenes, which are feasible in natural systems. In Chapter 5, the controls on vent fluid composition in a backare hydrothermal system are investigated. A comprehensive survey of the inorganic geochemistry of fluids from sites of hydrothermal activity in the eastern Manus Basin indicates that fluids there are influenced by input of acidic magmatic solutions at depth, and subsequently modified by variable extents of seawater entrainment and mixing-related secondary acidity production.