This　paper　considers　three　promotion　scenarios,　namely　seller-leading　promotion,　platform-leading　promotion,　and　no-leader　promotion,　and　reveals　the　optimal　strategy　and　decision　patterns　for　these　three　scenarios　using　a　stylised　model　and　lab　experiments.　We　focus　on　three　research　questions　for　the　seller　and　platform　as　follows:　(1)　Which　is　the　optimal　choice　between　unilateral　and　bilateral　promotion　under　different　promotion　scenarios?　(2)　How　will　the　promotion　scenarios　affect　their　joint　promotion?　(3)　To　a　seller/platform,　is　it　always　better　to　proactively　initiate　promotion　event?　Result　shows　that　the　optimal　promotion　strategy　varies　from　Seller　Promotion　Only　to　joint　promotion　and　then　to　Platform　Promotion　Only,　along　with　an　increasing　commission　rate.　Joint　promotion　is　optimal　only　when　the　commission　rate　is　moderate　and　when　the　product＇s　list　price　is　relatively　high.　The　promotion　scenario　affects　the　scope　and　intensity　of　joint　promotion　and　the　profit　of　the　entities.　The　joint　promotion　leader　has　the　first-mover　advantage,　which　helps　him/her　offer　fewer　coupons　but　obtain　higher　profits.　To　a　platform　or　seller,　launching　promotion　events　proactively　is　not　always　the　best　choice.　Experiments　show　that　the　subjects　behaved　as　predicted.　Moreover,　the　subjects　demonstrate　distinct　decision　patterns　under　different　promotion　scenarios.　Ultimately,　our　conclusions　can　provide　valuable　decision-making　guidance　to　marketing　managers　in　the　online　marketplace.　They　can　also　choose　appropriate　scenarios　on　the　basis　of　certain　promotion　objectives　with　knowing　decision　patterns　under　each　scenario.　©　2022　Elsevier　B.V.