The present study evaluates the hydrogen charging behaviour and the interaction between hydrogen and the microstructure of a UNS S31803 duplex stainless steel during electrochemical hydrogen charging. The two phase austenite/ferrite microstructure of duplex stainless steels complicates the study or prediction of the hydrogen susceptibility due to the large discrepancies between both phases in terms of their hydrogen properties. Austenite is characterized by a high hydrogen solubility together with a low hydrogen diffusivity, whereas ferrite shows a low hydrogen solubility and a high hydrogen diffusivity. Electrochemical hydrogen charging is performed galvanostatically in a sulphuric acid solution containing thiourea as a poison for the hydrogen recombination reaction. The total hydrogen content after melting is determined for various charging times with a thermal conductivity detector. Hydrogen saturation is reached after approximately ten days for a 0.3 mm thick duplex stainless steel plate. Continuous microstructural changes happening in the austenite phase fraction are observed, i.e. formation and interaction of dislocations, as a result of the high residual stresses inherent to the two phase microstructure of duplex stainless steels combined with the decreased shear modulus by the action of hydrogen. A theoretical saturation curve is constructed by solving Fick’s second law for one dimensional diffusion.