Because wet ultra-thin silicon oxides are extensively used in the microelectronic industry, we have investigated the growth of these oxides in various aqueous solutions using three main electrochemical techniques: (i) open circuit potential variation with time; (ii) linear voltammetry in a narrow range of potential; and (iii) electrochemical impedance spectroscopy under various polarisation potentials, to collect quantitative data regarding the growth kinetics of silicon oxide passivating layer, mainly at room temperature (r.t.). In oxidising alkaline solutions, the surface silicon oxide layer reached a limiting thickness value with time, related to oxidation/dissolution stationary behaviour. This observation was confirmed using ellipsometry. It was possible to reach with electrochemical techniques and ellipsometry the etching rate of the silicon substrate under the oxide layer in alkaline solution. Another interesting observation in this study was that the oxide layer showed a pronounced permeability to ions and oxidising agents in alkaline media, while this phenomenon vanished in acidic solutions.