Since the first facile synthesis of Ru(H2O)(6)(2) was published at the beginning of the eighties, several Ru-II aqueous inorganic and organometallic complexes were described, showing the increasing interest in this kind of compound due to potential applications in catalysis and medicine. To understand what governs the reactivity of the ligands in the first coordination sphere of the Ru-II center. we discuss its solution behavior, starting with the simplest aqueous species Ru(H2O)(6) and moving progressively in complexity with selected examples. In addition, we present variable-pressure measurements with two objectives to determine activation volumes for mechanistic assignments. and to shift equilibria by increasing the solubility of gaseous molecules.