Molybdenum disulfide (MoS2) and graphitic carbon nitride (g-C3N4) composite photocatalysts were prepared via a facile impregnation method. The physical and photophysical properties of the MoS2-g-C3N4 composite photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microcopy (HRTEM), ultraviolet-visible diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. The photoelectrochemical (PEC) measurements were tested via several on-off cycles under visible light irradiation. The photocatalytic hydrogen evolution experiments indicate that the MoS2 co-catalysts can efficiently promote the separation of photogenerated charge carriers in g-C3N4, and consequently enhance the H-2 evolution activity. The 0.5wt% MoS2-g-C3N4 sample shows the highest catalytic activity, and the corresponding H-2 evolution rate is 23.10 mu mol h(-1), which is enhanced by 11.3 times compared to the unmodified g-C3N4. A possible photocatalytic mechanism of MoS2 co-catalysts on the improvement of visible light photocatalytic performance of g-C3N4 is proposed and supported by PL and PEC results. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.