The CANDU Supercritical Water-Cooled Reactor’s extreme operating conditions and single-loop design have fuelled a need for better understanding of hydrothermal chemistry. This thesis reports the thermal stability and decomposition kinetics of perchloric acid in quartz and Pyrex® cells. HClO4 is an appropriate internal standard for Raman measurements of nickel(II) chloro complexes in quartz cells up to 200 ºC Raman spectroscopy. This thesis also reports the first Raman spectra for Ni2+, NiCl+ and NiCl2 from 8 to 120 ºC. Due to very weak bands and overlap of the contributing species, a thermodynamic speciation model, principle component analysis, and quantum mechanical predictions of the nickel(II) chloro Raman spectra were required to assign peaks. The assignment was confirmed by calculating temperature independent scattering coefficients from the spectra. This is the first study to obtain a spectrum for NiCl2 below 100 ºC.