Organic-rich diatomaceous ooze was reacted with sea water at 200 and 300°C, 500 bars, and an initial fluid/sediment mass ratio of 4 to evaluate the role of organic matter in sediment alteration processes at elevated temperatures and pressures. Decomposition of organic matter during the early stages of reaction dominated alteration and resulted in increased dissolved CO2, Corganic, NH4+ and H2S. Carboxylic acids were identified as reaction intermediaries during the transformation of organic matter to CH4 and CO2. Extensive SO42- reduction was observed at 300°C, but not at 200°C. Sediment alteration at 300°C resulted in recrystallization of calcic plagioclase and diatoms to an assemblage containing albite, pyrrohotite and quartz, in addition to minor anhydrite and Mg-smectite. In contrast, sediment alteration at 200°C resulted in only minor anhydrite and Mg-smectite formation, and there was no indication of recrystallization of diatoms to quartz. Rapid Mg-fixation during the early stages of alteration resulted in the complete removal of Mg from solution at 300°C and partial removal at 200°C. Production of acidity associated with Mg metasomatism was efficiently titrated by thermolytic and oxidative degradation of organic matter resulting in conspicuously low dissolved metal concentrations during the experiments. In the later stages of sediment alteration at 300°C, albitization of calcic plagioclase caused the continuous release of Ca to solution, while at 200°C fluid chemistry remained relatively constant. Speciation calculations for the 300°C experiment indicate an in-situ pH of 5.11 and a highly reduced redox state likely controlled by organic matter decomposition. Experimental results permit us to evaluate fluid-mineral equilibria in complex fluids containing high concentrations of dissolved organic species, CO2 and H2S, and constrain better alteration processes in sub-sea floor hydrothermal systems at sediment-covered spreading centers, such as the Guaymas Basin, Gulf of California.