The　conventional　understanding　of　facet　effects　on　photocatalysts　is　that　semiconductor　micro/nanocrystals　exposed　with　high-reactivity　facets　usually　exhibit　excellent　photocatalytic　activities.　However,　this　point　of　view　is　being　challenged　in　some　circumstances.　Herein,　two　types　of　facet-defined　AgCl　nanocrystals,　AgCl　nanocubes　exposed　with　six　{100}　facets　and　AgCl　octahedrons　enclosed　by　eight　{111}　facets　have　been　successfully　prepared.　The　comparative　studies　of　their　photocatalytic　properties　reveal　an　unusual　higher　photoreactivity　of　AgCl　nanocubes　than　that　of　AgCl　octahedrons,　although　AgCl　{100}　facets　have　lower　theoretical　surface　energy　than　the　AgCl　{111}　facets.　The　further　mechanism　investigation　suggests　that　the　much　decreased　reduction　potential　of　{111}　facet　would　result　in　insufficient　consumption　of　photo-excited　electrons　and　hence　depress　the　involvement　of　holes　in　photooxidation　reactions.　Consequently,　the　surface　electronic　structures　dominate　the　photoreactivities　of　AgCl　{111}　and　{100}　facets.　Our　work　reveals　the　true　photoreactivities　of　AgCl　{111}　and　(100)　facets　by　direct　experimental　evidence,　and　the　proposed　mechanism　will　facilitate　further　development　of　highly　efficient　AgCl　photocatalysts.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.