[en] Heterostructured Mo${}_2$C-MoO${}_x$ on carbon cloth (Mo${}_2$C-MoO${}_x$/CC), as a model of easily oxidized electrocatalysts under ambient conditions, is investigated to uncover surface reconfiguration during the hydrogen evolution reaction (HER). Raman spectroscopy combined with electrochemical tests demonstrates that the Mo${}^{VI}$ oxides on the surface are in situ reduced to Mo${}^{IV}$, accomplishing promoted HER in acidic condition. As indicated by density functional theoretical calculations, the in situ reduced surface with terminal Mo=O moieties can effectively bring the negative ΔG${}_{H^{\ast <!--\; ???\; -->}}$ on bare Mo${}_2$C close to a thermodynamic neutral value, addressing difficult H${}^{\ast <!--\; ???\; -->}$ desorption toward fast HER kinetics. The optimized Mo${}_2$C-MoO${}_x$/CC only requires a low overpotential (η${}_{10}$) of 60 mV at −10 mA cm${}^{\text{â<!-- ?? -->}\text{ˆ<!-- ?? -->}\text{’<!-- ??? -->}2}$ in 1.0 m HClO${}_4$, outperforming Mo${}_2$C/CC and most non-precious electrocatalysts. In situ surface reconfiguration are shown on W${}_2$C-WO${}_x$, highlighting the significance to boost various metal-carbides and to identify active sites. (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)