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• Understanding catalysts structural/electronic properties under operating conditions. • Elucidating Fe doping's role in iron-molybdenum sulfide electrocatalysts. • Identification of active species in HER molybdenum sulfides. • Understanding HER activity promoted by iron-doped and non-doped MoS x. • X-ray absorption spectroscopy investigation of amorphous molybdenum sulfides. 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 MoS x 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. [ABSTRACT FROM AUTHOR]