Methods are described for determining equilibria and reactions in H 2 O 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 CO 2 , N 2 O, and NO 3 -under these conditions. The CO 3 2-, HCO 3 -, CO 2 , and NH 2 CO 2 -components of aqueous (NH 4 ) 2 CO 3 were observed directly and reveal a preference for neutral species (CO 2 and NH 3 ) at higher temperature. The exothermic decomposition of aqueous [NH 3 -OH]NO 3 (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 Damköhler 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.