Abstract This paper covers one of the building blocks required to complete a mechanistic CO2 / H2S corrosion model: electrochemistry of carbon steel CO2 corrosion in the presence of traces of H2S (3-250 ppm in the gas phase). Electrochemical impedance spectroscopy and linear polarisation resistance were used to investigate the mechanism of CO2/H2S corrosion in a glass cell at room temperature, at atmospheric pressure, pH5, using a rotating cylinder electrode at 1000 rpm. Very thin sulphide film was formed on the steel electrode in all experiments with H2S. At low concentrations (<15 ppm) the sulphide films reduced the corrosion rate while at higher concentrations of H2S (up to 250 ppm) the trend was reversed and a mild increase of the corrosion rate was seen.