We　report　the　assembly　of　nanosized　ZnS　particles　on　the　2D　platform　of　a　graphene　oxide　(GO)　sheet　by　a　facile　two-step　wet　chemistry　process,　during　which　the　reduced　graphene　oxide　(RGO,　also　called　GR)　and　the　intimate　interfacial　contact　between　ZnS　nanoparticles　and　the　GR　sheet　are　achieved　simultaneously.　The　ZnS-GR　nanocomposites　exhibit　visible　light　photoactivity　toward　aerobic　selective　oxidation　of　alcohols　and　epoxidation　of　alkenes　under　ambient　conditions.　In　terms　of　structure-photoactivity　correlation　analysis,　we　for　the　first　time　propose　a　new　photocatalytic　mechanism　where　the　role　of　GR　in　the　ZnS-GR　nanocomposites　acts　as　an　organic　dye-like　macromolecular　＂photosensitizer＂　for　ZnS　instead　of　an　electron　reservoir.　This　novel　photocatalytic　mechanism　is　distinctly　different　from　all　previous　research　on　GR-semiconductor　photocatalysts,　for　which　GR　is　claimed　to　behave　as　an　electron　reservoir　to　capture/shuttle　the　electrons　photogenerated　from　the　semiconductor.　This　new　concept　of　the　reaction　mechanism　in　graphene-semiconductor　photocatalysts　could　provide　a　new　train　of　thought　on　designing　GR-based　composite　photocatalysts　for　targeting　applications　in　solar　energy　conversion,　promoting　our　in-depth　thinking　on　the　microscopic　charge　carrier　transfer　pathway　connected　to　the　interface　between　the　GR　and　the　semiconductor.　©　2012　American　Chemical　Society.