Electrochemical hydrogen absorption in duplex steels is not fully understood. In this work, an in-situ synchrotron cross-sectional X-ray micro-diffraction analysis is performed on steel with comparable phase fractions of ferrite and austenite, coupled with electrolytic hydrogen charging. The results reveal that charging with a constant current density of 10 mA/cm² for 5 h leads to expanding the austenitic lattice to a depth of approximately 250 µm, up to ≥0.15 %. In contrast, the lattice parameter of the ferrite phase remains unchanged during this process. As the austenite expansion progresses, it generates different amounts of equivalent in-plane compressive stresses, which amount to approximately -150 and -450 MPa in the austenite and ferrite phases at the sample surface, respectively. Using a finite element model of grain interaction, this difference is qualitatively interpreted by mutual mechanical constraints between ferrite and austenite, as well as between the hydrogen-charged surface layer and the underlying material.