The basis for the superior performance of CoMo6S24@NU1K relative to bulk CoMo6Sx almost certainly includes the much higher areal density of the former. A close-packed monolayer of flat Anderson-type PTMs on a flat electrode amounts to about 6 x 10 -10 mol/cm 2 -perhaps a reasonable lower limit estimate of the number of exposed active-sites for a nonporous, film-like monolith of bulk CoMo6Sx, For CoMo6S24@NU1K the real density PTM units is 500× higher, when loaded as described in the Supplemental Information. It seems likely that the redox conductivity of CoMo6S24@NU1K is insufficient to enable active-sites far from the PTM@MOF/electrode interface to participate electrocatalytically to the extent that those close to the interface participate. Even semi-quantitative assessment will require charge-transport studies that are beyond the scope of this initial paper. Nevertheless, if we take 500-fold as an upper limit for the increase in fully participating electrocatalytic active-sites for electrode-immobilized CoMo6S24@NU1K relative to electrode-immobilized, bulk CoMo6Sx, and if we assume a Tafel slope of 120 mV•dec -1 , the loading difference translates to a decrease in HER overpotential of ~320 mV at current densities of 10 mA cm - 1 . An arguably more realistic ratio of fully addressable HER active-sites for electrode-immobilized CoMo6S24@NU1K versus electrode-immobilized, bulk CoMo6Sx, is 20. This catalyst areal loading capacity translates to a decrease in HER overpotential of 156 mV. Regardless, the high arealconcentration of PTM-based active-sites appears, on its own, to confer a substantial decrease in HER kinetic overpotential. This observation underscores a key potential advantage of presenting active-sites via MOF-installation, compared with simply immobilizing the same catalyst on a flat electrode at monolayer density. No doubt, other factors are important, but their assessment will require much more detailed kinetics studies. Finally, it may be useful to frame electrocatalytic activity in terms of local turnoverfrequency (TOF). If, for simplicity, we assume one activesite per PTM in a CoMo6S24@NU1K coating and a kinetically effective loading equivalent to 500 monolayers of reactantaccessible PTM, we obtain a TOF ~1 s -1 at an iR-corrected overpotential 100 mV. If the electrocatalytically effective areal concentration is equivalent to, say, 20 monolayers of PTM, the effective TOF for the small fraction of sites actively participating, would be ~25 s -1 .