A　series　of　vented　hydrogen-air　explosion　experiments　were　carried　out　in　an　end-vented　rectangular　tube,　and　the　effect　of　hydrogen　concentrations　varying　from　10%　to　50%　on　the　flame　evolution　and　the　maximum　overpressure　inside　and　outside　the　duct　were　investigated.　Experimental　results　reveal　that　the　flame　behavior　was　strongly　affected　by　hydrogen　concentration:　the　flame　speed　at　the　vent　exit　first　increased　and　then　decreased　as　hydrogen　concentration　was　increased;　a　tulip　flame　and　two　bright　fireballs　were　respectively　observed　within　and　outside　the　duct　in　some　tests.　Two　dominant　pressure　peaks　and　two　types　of　oscillations　could　be　distinguished　in　the　internal　pressure　curves,　corresponding　to　the　rupture　of　the　membrane,　venting　of　the　burned　gas,　Helmholtz,　and　acoustic　oscillations,　respectively.　Under　the　current　experimental　conditions,　the　pressure　peak　with　a　low　amplitude　owing　to　the　rupture　of　membrane　dominated　the　external　pressure-time　histories　for　hydrogen　concentrations　less　than　25%.　However,　when　the　hydrogen　concentration　ranged　from　30%　to　50%,　the　pressure　peak　caused　by　the　second　external　explosion　become　dominant.　Besides,　the　influence　of　the　external　explosions　on　the　internal　pressure　peak　structures　was　discussed.　As　hydrogen　concentration　was　increased,　the　maximum　internal　overpressure　first　increased　and　then　decreased,　whereas　the　maximum　external　overpressure　increased　monotonically.　In　this　study,　the　maximum　reduced　overpressures　calculated　by　Molkov＇s　best-fit　model　agreed　well　with　the　experimental　data　for　near-stoichiometric　and　rich　hydrogen-air　mixtures.(c)　2022　Published　by　Elsevier　Ltd　on　behalf　of　Institution　of　Chemical　Engineers.