Canada's research and development efforts toward the CANDU-Supercritical-Water Cooled Reactor have fuelled the need for hydrothermal chemistry research in the "transition" region from 250 °C to 450 °C. In this thesis, a thermochemical model was developed to describe iron transport and hydrolysis, based on magnetite and hematite solubility data obtained below 300 °C. Extrapolations of solubility and complexation reactions into the supercritical region with two different pH control agents, LiOH and NH3 found that the anionic species are a dominant factor in determining magnetite solubility. The effects of the chloride ion and ammonia on nickel speciation were studied by Raman spectroscopy up to 150 °C. The stepwise chloride complexes NiCln (2-n)+, (n = 1-4), were all identified in the spectra. The monochloro complexes and triple ions were found to dominate above 80 °C. Nickel ammonia complexes were observed but were found to decrease in importance as temperature was increased. Experiments which show promise for extending these studies to higher temperatures, and to iron (II) and iron (III) species are identified.