In　this　study,　experiments　on　the　duct-vented　deflagrations　of　hydrogen–methane–air　mixtures　were　performed　in　a　cylindrical　vessel　connected　with　a　relief　duct,　at　an　initial　pressure　of　101　kPa　and　an　initial　temperature　of　290　K,　to　investigate　the　effects　of　equivalence　ratio　(),　in　the　range　of　0.6–1.8,　on　flame　evolution　and　pressure　buildup　within　and　outside　the　venting　configuration.　Experimental　results　reveal　that　secondary　explosion　in　the　relief　duct　occurred　and　a　pressure　peak　with　amplitude　higher　than　the　overpressure　in　the　vented　chamber　formed,　which　resulted　in　the　gases　in　the　relief　duct　flowing　reversely　to　the　vented　chamber　(reverse　flow).　The　secondary　explosion　in　the　relief　duct　reduced　the　venting　efficiency　in　the　vented　chamber　and　created　a　pressure　peak　there,　which　dominated　the　pressure–time　histories　in　the　vented　cylindrical　vessel　for　ranging　from　0.8　to　1.4.　Acoustic　oscillations　of　the　overpressure　in　the　cylindrical　vessel　could　be　easily　distinguished　for　≤　1.0,　and　the　pressure　peak　owing　to　acoustic　enhanced　combustion　was　dominant　at　=　0.6.　The　maximum　overpressure　in　the　cylindrical　vessel　first　increased　and　thereafter　decreased　as　was　increased　from　0.6　to　1.5　and　was　independent　of　for　richer　mixtures.　A　shock　wave　was　visualized　in　the　tests　with　in　a　range　of　1.0–1.2　when　the　combustible　cloud　outside　the　venting　configuration　was　ignited,　which　resulted　in　a　pressure　spike　in　the　external　pressure–time　histories.　The　maximum　overpressure　outside　the　relief　duct　first　increased　and　thereafter　decreased　as　was　increased.　©　2021