This dissertation is composed of two main topics related to high-temperature high-pressure water radiolysis, the reactions of hexaaquo transition metal ions and radiolysis suppression by hydrogen injection. The introductory chapter provides the fundamental materials of all chapters.  Chapter 2 investigates 'The Divalent-to-Monovalent Redox Potentials of Hexaaquo Transition Metal Ions'. The Marcus Theory of electron transfer has been applied to estimate the redox potentials of hexaaquo transition metal ions. A number of coordination compounds whose redox potentials and self-exchange rate constants are well-established have been used as references. The rate constants for the cross-reactions of the transition metal ions with the selected coordination compounds have been carefully measured by means of pulse radiolysis. The calculation using the Marcus Cross Relation fails to give sensible results. The reasons for this failure are discussed. Chapter 3 investigates 'Electron Transfer in the Reactions of Hexaaquo Transition Metal Ions with the Hydrated Electron'. The rate constants for the reactions of divalent hexaaquo transition metal ions with the hydrated electron have been measured. The reaction rates have been found to be affected by the ionic strength of the ionic systems. None of the reactions studied are limited by the diffusion rate of the reacting species. All of the reactions studied exibit the Arrhenius behavior at temperatures up to 300oC. The Marcus Theory has been applied to estimate the range of the reorganization energies of the electron transfer reactions.  Chapter 4 investigates 'The Reactions of Hexaaquo Transition Metal Ions: Nickel(I) and Zinc(I) with Radiolytic Oxidizing Species'. The reactions of the monovalent metal ions with the radiolytic oxidizing species are responsible for the decays of the transient species in pure water. The study in aqueous methanol has revealed that the Ni+(aq) ions do not undergo bimolecular disproportionation, whereas, the Zn+ (aq) ions decay via this reaction forming metallic zinc with a relatively slow rate constant. Applying the Smoluchowski equation, it is apparent that all of the reactions studied are not limited by the diffusion of the reactants, one exception being the reaction of Ni+(aq) with ●OH radicals at room temperature. The recombination reaction of hydroxymethyl radicals has been investigated as part of the kinetics model used in the aqueous methanol solutions. The high-temperature optical spectra of Ni+(aq) , Zn+(aq) and ●CH2OH radical have been recorded.  Chapter 5 investigates 'The Critical Hydrogen Concentration in Elevated Temperature and Supercritical Water'. The Critical Hydrogen Concentration (CHC) for suppression of water radiolysis has been studied at temperatures up to the supercritical regime in Hastelloy and sapphire tubing. The high CHC results in Hastelloy tubing have suggested that a radiation-induced corrosion process may take place in this material under the experimental conditions. The experimental results obtained from the sapphire system have been used to estimate some rate constants needed for water radiolysis modeling.