A　relief　duct　is　often　used　for　indoor　vented　enclosures　to　discharge　unburnt　and　burnt　gas　mixtures　to　outdoors,　but　its　use　can　significantly　affect　pressure　relief　and　flame　behavior.　In　the　current　paper,　duct-vented　explosions　of　hydrogen-air　mixtures　with　equivalence　ratios　ranging　from　0.8　to　4.0　were　experimentally　investigated　in　a　small　cylindrical　vessel　with　a　50-cm-long　relief　duct.　Different　from　＂coherent　deflagration＂　as　previously　reported,　the　maximum　pressure　rise　rate　in　the　duct　is　always　larger　than　that　in　the　vessel.　Three　pressure　peaks　can　be　identified　on　the　external　pressure　profile,　which　result　from　the　burst　of　the　vent　cover,　the　burn-up　in　the　duct,　and　the　external　explosion,　respectively.　Whether　the　second　or　the　third　pressure　peak　is　the　dominant　one　depends　on　the　hydrogen　equivalence　ratio.　Compared　with　simple　vented　explosions,　the　use　of　a　relief　duct　decreases　the　pressure　relief　efficiency　and　the　external　flame　size　for　all　equivalence　ratios　tested.　©　2016　American　Chemical　Society.