Ruthenium chloride and iridium chloride precursors (denoted hereafter as RuCl3·aH2O and IrCl3·bH2O) reacted with water molecules and transformed into the chloro-oxy-hydroxyl-metal (denoted as Ru(OH) δ Cl3−δ ·cH2O and Ir(OH) ω Cl3−ω ·dH2O) structures when they were dissolved in water. The storage time, preheating, and initial pH of the deposition baths were demonstrated to significantly influence the structures of these dissolved precursors. The rate of hydrous oxide deposition by means of cyclic voltammetry was predominantly determined by the structures of the dissolved precursors and the initial pH of the deposition bath, which also caused a variation in the performance (i.e. specific capacitance) of the hydrous oxides (i.e. RuO x ·nH2O and (Ru+Ir)O y ·mH2O) for the application of electrochemical supercapacitors.