The comprehensive characterization of the structure-electrocatalytic activity relationships is of great interest to support the development of efficient catalysts for hydrogen evolution reaction (HER) in proton-exchange membrane (PEM) electrolyzers. The successful implementation of a bimetallic iron-molybdenum sulfide electrocatalyst for HER into a PEM electrolyzer has been reported. However, the precise nature of their active sites is still elusive. In this study, we employed X-ray absorption spectroscopy (XAS) to study iron-molybdenum and molybdenum sulfides obtained by a microwave irradiation synthetic approach. Operando XAS measurements in an acidic environment revealed sulfur-defective amorphous MoSx and FeS phases. Although the amount of dopant Fe is 10 times lower than that of Mo, the synergy between the two phases promotes the HER performance of iron-molybdenum sulfides.