In　this　study,　the　effects　of　hydrogen　ratio　(χ)　on　the　vented　deflagration　of　stoichiometric　hydrogen/methane/air　mixtures　were　investigated　in　a　cylindrical　vessel　connected　with　a　relief　duct　at　an　initial　pressure　of　100　kPa　and　an　initial　temperature　of　290　K.　Flame　behavior　and　pressure　evolution　in　the　vessel,　the　duct　and　the　free　field　outside　the　vented　configuration　were　clarified　by　means　of　pressure　measurement　synchronized　with　high-speed　photography.　Experimental　results　revealed　that　the　secondary　explosion　in　the　relief　duct　significantly　affected　the　venting　process　because　it　created　a　negative　pressure　gradient　and　consequently　a　reverse　flow　regardless　of　χ.　The　pressure　peak　in　the　relief　duct　resulting　from　the　secondary　explosion　was　higher　than　the　maximum　explosion　overpressure　in　the　vessel　when　χ　＞　0.8,　and　a　shock　wave　in　the　relief　duct　was　visualized　in　these　tests.　Various　pressure　peaks　in　the　pressure–time　histories　in　the　vessel　owing　to　vent　failure,　reverse　flow,　and　acoustically　enhanced　combustion　could　be　distinguished　when　χ　≤　0.8,　and　the　one　which　was　dominant　depended　on　χ.　Only　one　pressure　peak　formed　when　χ　＞　0.8.　The　average　maximum　explosion　overpressure　in　the　vessel　increased　monotonically　from　26　kPa　to　289　kPa　as　χ　increased　from　0　to　1.0.　The　maximum　rate　of　pressure　rise　in　the　vessel　approximated　that　in　the　duct　when　χ　≤　0.45,　but　the　former　was　much　lower　than　the　latter　in　the　tests　with　higher　χs.　The　average　maximum　overpressure　outside　the　vented　configuration,　owing　to　external　explosion,　increased　from　several　kPa　to　62　kPa　with　χ　increasing　from　in　0　to　1.0,　and　a　shock　wave　downstream　of　the　relief　duct　formed　when　χ　＞　0.8.　©　2022　Elsevier　Ltd