The knowledge of the quinary Pb–Bi–O–Fe–Hg is necessary for understanding the degradation mechanisms of the T91 steel used as structural material in future ADS nuclear reactors. In this device, the steel will be in direct contact with the liquid spallation target (which is constituted by lead or lead-bismuth eutectic) surrounded by a reduced oxygen pressure atmosphere. In the present work, the characterization of the pseudo-binary PbO–Fe2O3 cut has been performed. In order to complete the available data in the literature, some experimental investigations by DTA, isothermal annealing, SEM and EPMA have been done. These results have allowed proposing a thermodynamic assessment using the Calphad method by the ThermoCalc software.