The voltammetric formation and potentiostatic growth of anodic films on Ru/Pt electrodes in HClO4 and HCI solutions were studied by single negative potential sweeps and cathodic charging curves in the potential range from -0.25 to 1.1 V vs SCE. The growth of the anodic layer proceeds through the formation of two layers of different reduction reversibility. At potentials below 500 mV, the layer more reversibly reduced, grows slowly to a maximum coverage equivalent to one oxygen monolayer. The thicker, and more stable, layer increases with holding time to a maximum of about three oxygen monolayers during the period of time studied (7 h). At holding potentials above 500 mV, the reduction charge of the anodic layer reaches a constant value after polarization for 1 h. Growth starts with formation of two layers which, with time, become a single layer which is hardly reducible. The results suggest the eventual formation of anhydrous RuO2. In HCl solutions, Cl- adsorption inhibits the formation of the anodic layer, decreasing its growth rate but reaching no limiting thickness for 7 h. At holding potentials below 650 mV vs SCE, only a single layer is formed with slight structural changes. At potentials above 650 mV, the initially homogeneous film converts with holding time into a bilayer where the outer layer becomes hardly reducible. This layer is assumed to be a stable anionic hydroxy species (RuCl5OH2-) which dissolves as Ru2O2Cl6(H2O)2-. In HClO4 and HCl the layer growth follows a direct logarithmic law.