Methods are described for determining equilibria and reactions in H2O by FT Raman spectroscopy and stainless steel and titanium flow cells operated up to 500 K and 275 bar. Semiquantitative correlations were achieved between the Raman scattering and concentration of CO2, N2O, and NO3- under these conditions. The CO32-, HCO3-, CO2, and NH2CO2- components of aqueous (NH4)(2)CO3 were observed directly and reveal a preference for neutral species (CO2 and NH3) at higher temperature. The exothermic decomposition of aqueous [NH3-OH]NO3 (HAN) was investigated at a pressure of 275 bar as a function of temperature, concentration, and flow rate. The flow reactor appears to be most useful when the Damkohler number is 1-2. From the induction times-to-exotherm in the Ti cell, apparent activation energies of 129 +/- 29 kJ/mol for 0.87-1.52 m HAN and 66 +/- 8 kJ/mol for 1.58-1.74 m HAN were obtained. Arrhenius preexponential factors are estimated. The apparent activation energies are compared to previous estimates at different conditions, and are consistent with the formation of a critical concentration of a species which catalyzes the exothermic process.