This work shows how a molecular Mo3S4 cluster bonded to a photoelectrode surface via a phosphonate ligand can be a highly effective co-catalyst in photocathodic hydrogen evolution systems. Using a TiO2 protected n(+)p Si photocathode, H-2 evolution occurs with an onset of +0.33 V vs. RHE in an acid solution with this precious metal-free system. Using just the red part of the AM1.5 solar spectrum (lambda > 635 nm), a saturation current of 20 mA/cm(2) is achieved from an electrode containing Mo3S4 dropcasted onto a 100 nm TiO2/7 nm Ti/n(+)p Si electrode. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.