The aim of this work is to develop an efficient and stable transparent electrode for the reduction of dissolved oxygen, to be used for characterising biofilm formation in seawater and scale deposit, both by optical observations and electrochemical experiments. It was achieved by depositing a transparent conductive tin dioxide film on a glass substrate and modifying the surface by a very small gold charge. A reduction kinetics of dissolved oxygen as fast as that for a massive gold electrode has been obtained with a few monolayer only equivalent gold deposit via a simple chemical treatment. The long term stability has been checked, opening the way for practical applications. Electrochemical impedance spectroscopy was used to analyse the working conditions of the modified electrode. It is shown that SnO2 is operating in the cathodic regime under weak depletion conditions with a surface-state mediated charge transfer.