In　this　study,　experiments　on　the　vented　deflagration　of　hydrogen–methane–air　mixtures　were　performed　in　a　vertical　rectangular　duct　with　a　vent　at　its　top　end.　A　nondimensional　vent　coefficient　(Kv)　was　employed　to　characterize　the　effects　of　vent　area　(Av)　on　flame　evolution　and　pressure　buildup　within　and　outside　the　duct.　The　maximum　internal　overpressure　was　attained　near　the　duct　bottom　for　a　certain　Kv.　As　Kv　was　increased　from　2.2　to　11.9,　the　maximum　internal　overpressure　near　the　vent　and　at　the　center　of　the　duct　increased　monotonically;　however,　a　nonmonotonic　trend　was　observed　at　the　duct　bottom.　Helmholtz　oscillations　of　the　internal　flame　and　overpressure　were　observed　in　the　tests　for　low　Kv,　and　acoustic　oscillations　of　the　internal　overpressure　with　a　frequency　of　~1200　Hz　appeared　in　all　tests.　The　maximum　external　overpressure　first　increased　and　then　decreased　with　an　increase　in　Kv.　The　external　explosion　induced　a　pressure　peak　in　the　duct,　and　the　effect　of　external　explosion　on　the　venting　of　the　internal　deflagrations　was　weakened　as　Kv　≥　7.8.　©　2020　Hydrogen　Energy　Publications　LLC