Experiments　on　duct-vented　explosions　of　hydrogen–air　mixtures　in　a　12.3　l　cylindrical　vessel　were　conducted,　and　the　effects　of　duct　length　and　hydrogen　concentration　on　the　maximum　overpressure　and　flame　behavior　within　and　outside　the　vented　enclosure　were　investigated.　The　results　show　that　the　maximum　overpressure　in　the　vessel　first　increased　and　then　was　maintained　nearly　unchanged　with　the　length　of　a　relief　duct　increasing　to　2　m.　For　a　given　duct　length,　the　maximum　overpressure　first　increased　and　then　decreased　when　hydrogen　concentration　increased　from　20%　to　55%.　The　burn-up　in　the　duct　caused　the　gas　mixtures　to　move　in　reverse　from　the　duct　to　vessel,　which　consequently　decreased　the　venting　efficiency.　A　pressure　wave　caused　by　burn-up　in　the　duct　was　observed,　which　resulted　in　a　pressure　peak　in　the　external　pressure–time　histories　after　it　traveled　outside　the　duct.　The　maximum　external　overpressure　first　increased　and　then　decreased　with　an　increase　in　duct　length.　For　a　given　duct　length,　the　maximum　external　overpressure　increased　with　an　increase　in　hydrogen　concentration.　©　2018　Hydrogen　Energy　Publications　LLC