The simplest interpretation of a curved Tafel plot for CoMo6S24@NU1K is that the mechanism for HER, or at least the rate-determining step, changes as function of electrochemical driving force. A more prosaic interpretation is that CoMo6S24@NU1K is poorly conductive, and that the potential-dependence of the HER rate (current) is dominated by the ohmic resistance of the catalyst itself. Note that instrument-based iR corrections cannot compensate for catalyst resistance. Follow-up studies will be needed in order to gauge the importance of this effect. The observed Tafel slope at low overpotential is suggestive of a Volmer-Tafel mechanism where two electrons are transferred before rate-limiting chemical reactivity between pairs of sorbed H atoms, presumably sited on catalyst sulfur ions. An intriguing and speculative alternative, however, is rapid transfer of a pair of electrons to a proton to yield a Mo IV -hydride, followed by rate-determining H2 formation from the hydride and a sulfidesited proton. We have shown elsewhere that when single Mo(SH)2 units are grafted to the hexa-zirconium(IV) node of NU1K via Mo-O-Zr and Mo-(OH)-Zr linkages, yielding Mo(IV) in a distorted tetrahedral environment, electrocatalytic hydrogen evolution occurs via Mo(IV) reduction to Mo(II) followed proton recruitment to form Mo IV -hydride, and rate-determining reaction of the hydride with a sulfursorbed proton. Thus, in the single-metal-ion limit, the molybdenum-sulfur catalyst generates H2 via a mechanism reminiscent of the behavior of various hydrogenases. For the mechanism to be viable with a PTM, loss of a sulfide, sulfhydryl, or disulfide ligand, and exposure of a molybdenum site would be required. While further studies will be needed to gauge catalyst evolution and pin-down the HER mechanism, preliminary computational studies support the notion of labialization of sulfur species upon reduction of molybdenum.