We　experimentally　investigated　the　pressure　buildup　and　flame　behavior　during　the　vented　deflagration　of　hydrogen-air　mixtures　with　concentrations　ranging　from　13%　to　39%　that　were　centrally　ignited　in　a　1-m3　rectangular　vessel　with　a　500　mm　×　400　mm　top　vent.　The　performance　of　some　available　models　for　estimating　the　maximum　reduced　overpressure　was　experimentally　evaluated.　The　maximum　reduced　overpressure　increased　from　approximately　3　kPa　to　100　kPa　as　hydrogen　concentration　increased　from　13　to　39%.　Turbulent　pressure　oscillations　with　frequencies　of　200–300　Hz　triggered　by　external　explosions　were　observed　in　our　tests　with　22–39%　hydrogen-in-air　mixtures.　Molkov＇s　best-fit　and　conservative　models　predict　the　maximum　reduced　overpressure　well　for　lean　and　rich　hydrogen　mixtures,　respectively.　The　average　speed　of　the　external　flame　first　decreases　to　a　minimum　value　with　the　fireball　expanding　to　its　maximum　size　and　then　increases.　As　hydrogen　concentration　increases,　the　maximum　length　and　duration　of　the　external　flame　increases　and　decreases,　respectively.　©　2018　Hydrogen　Energy　Publications　LLC