The results of experiments examining the thermal decomposition of N2O and its reactivity with methane in supercritical water at approximately 500 degrees C and 30MPa are presented. The rate of thermal decomposition is observed to be close to the rate predicted by extrapolating an Arrhenius expression from the literature that has been shown to be valid at 750 degrees C and 1.0 MPa. The observed first-order rate constant at 500 degrees C is 9.4 x 10(-6) s(-1). There is no significant effect on N2O stability due to the presence of supercritical water relative to ambient pressure. Measurements exploring the conversion rate of methane in the presence of N2O reveal that simple oxidation chemistry competes with polymerization. The data suggest that much of the carbon in the system is converted to (CH2)(n) oligomers that separates from the supercritical phase. A detailed kinetic mechanism is used to explore characteristics of these competing processes.