A representative metal-organic framework, NU-1000, was functionalized with MoSx. The previously determined crystal structure of the material, named MoSx-SIM, consists of monometallic Mo(IV) ions with two sulfhydryl ligands. The metal ions are anchored to the framework by displacing protons presented by the -OH/-OH2 groups on the Zr-6 node. As shown previously, the MOF-supported complexes are electrocatalytic for hydrogen evolution from acidified water. The earlier electrocatalysis results, together with the nearly ideal formal potential of the Mo(IV/II) couple (i.e., nearly coincident with that of the hydrogen couple), and the physical proximity of UV-absorbing MOF linkers to the complexes, suggested to us that the linkers might behave photosensitizers for catalyst reduction, and subsequently, for H-2 evolution from water. To our surprise, MoSx-SIM, when UV-illuminated in an aqueous buffer at near-neutral pH, displays a biphasic photocatalytic response: an initially slow rate of reaction, i.e. 0.56 mmol g(-1) h(-1), followed by an increase to 4 mmol g(-1) h(-1). Ex-situ catalyst examination revealed that nanoparticulate MoSx suspended within the reaction mixture is the actual catalyst. Thus, photo-assisted restructuring and detachment of the catalyst or pre-catalyst from the MOF node appears to be necessary for the catalyst to reduce water at neutral (C) The Author(s) 2019. Published by ECS.