We find that a reactant-accessible, agglomerationimmune, polythiometalate -CoMo6S24@NU1K -can be obtained in quantitative yield by isolating the corresponding Anderson POM, in pore-size-matching fashion, within a hierarchically porous MOF and then heating the ensemble under flowing H2S to exchange oxygen for sulfur. The resulting PTMs are unencumbered by capping ligands, but present a rich sulfur environment comprising S 2-, SH -, and S2 2-. With the majority of the cluster's sulfur ions accessible to candidate reactants, high catalytic, electrocatalytic, and/or photocatalytic activity for suitably chosen reactions might be expected. Indeed, when assembled as thin-films on electrodes, CoMo6S24@NU1K samples are competent for hydrogen evolution from aqueous acid. The films display kinetic overpotentials of just 100 mV at a benchmark current density of 10 mA/cm 2 . The high activity accrues, at least in part, from the ability of the hybrid material to present the equivalent of ca. 500 monolayers of closed-packed PTM clusters, but in a mechanically, chemically, and thermally stable matrix featuring ample molecular-scale porosity.