The influence of hydrogen on the microstructure of austenitic-ferritic duplex stainless steel (DSS) was investigated. The thin foils after cathodic hydrogen charging were examined using transmission electron microscopy (TEM). The effect of various experimental conditions (i.e., current density, type of the electrolyte and presence of the hydrogen recombination poison) on the microstructure evolution during cathodic hydrogen charging was studied. The characteristic changes of the alpha and gamma phases were observed. The microstructural changes in the gamma phase consist in an increase of stacking faults and dislocations density. The possible explanations of the effects observed were discussed. It was concluded that the appropriate choice of the experimental conditions permits us to control the microstructure evolution rate resulting from the hydrogenation process. (C) 2003 Elsevier Science B.V. All rights reserved.