The word “hydrolysis,” literally “water splitting,” refers here to those reactions of metallic ions with water that liberate protons and produce hydroxy or oxy complexes in solution and precipitate hydroxide or oxide solids. Over the wide range of pH available in water, the strong tendency of hydroxide to bond to and bridge among cations leads to a multitude of species for many of the metals. Properties of the metal ion such as “hardness” and, especially, charge and size, determine the strength of these interactions and the species formed by each. Over 200 mononuclear species have been identified that predominate in dilute solutions across the pH scale. It is the stability of these species that largely determines the pH dependence of the solubility of the oxide or hydroxide solid formed. Both the tendency of the variety of polynuclear species (those containing more than one metal ion) to form and the solubility of the hydrolytic solid correlate with the stability of the first mononuclear species. Other correlations allow one to predict the enthalpy and entropy of a variety of hydrolysis reactions, and therefore the dependence of the stability of hydrolysis products on temperature.