Reduced　graphene　oxide　(RGO)/ZnIn2S4　nanocomposites　with　RGO　in　different　reduction　degrees　have　been　prepared　and　their　photocatalytic　performance　for　H2　evolution　under　visible　light　irradiation　has　been　evaluated.　The　reduction　degree　of　RGO　in　the　RGO/ZnIn　2S4　nanocomposite　synthesized　via　a　direct　hydrothermal　process　was　low　and　the　resultant　RGO/ZnIn2S4-hydrothermal　exhibited　lower　photocatalytic　performance　for　H2　evolution　even　when　compared　to　pure　ZnIn2S4.　The　reduction　degree　of　RGO　in　the　RGO/ZnIn2S4　nanocomposite　can　be　increased　by　a　further　photoreduction　or　hydrazine　reduction.　These　highly　reductive　RGO/ZnIn2S4　nanocomposites　showed　significantly　enhanced　photocatalytic　activity　for　H2　production　under　visible　light　irradiation.　Highly　reduced　RGO　in　the　RGO/ZnIn2S4　nanocomposites　serves　as　a　good　electron　acceptor/mediator　as　well　as　the　H　2　evolution　site　to　promote　the　photocatalytic　water　splitting　over　ZnIn2S4.　A　correlation　between　the　photocatalytic　performance　for　H2　evolution　and　the　RGO　reduction　degree　over　RGO/ZnIn2S4　nanocomposites　was　shown　for　the　first　time.　This　study　provides　new　insights　in　the　developments　of　RGO-based　nanocomposite　materials　for　photocatalytic　applications.　©　2013　The　Royal　Society　of　Chemistry.