The　effect　of　the　static　burst　pressure　of　the　vent　cover　(pv)　on　the　flame　evolution　of　hydrogen–methane–air　deflagration　and　pressure　buildup　inside　and　outside　the　duct　was　studied.　A　series　of　vented　deflagration　experiments　of　hydrogen-methane-air　mixtures　were　carried　out　in　a　300　mm　long,　300　mm　wide,　and　1000　mm　high　duct　with　a　250　mm　×　250　mm　vent　at　the　top.　Stoichiometric　hythane–air　mixtures　were　prepared　according　to　Dalton’s　law　of　partial　pressure.　The　mixed　gas　was　ignited　at　the　vessel　center　by　an　electric　spark.　The　explosion　flames　were　recorded　by　a　high-speed　camera　at　2000　frames　per　second.　Piezoresistive　pressure　sensors　were　used　to　record　the　internal　and　external　overpressures.　Experimental　results　reveal　that　pv　significantly　affects　the　pressure-time　histories　and　the　flame　propagation　in　the　duct.　Helmholtz-type　oscillations　of　the　internal　pressure　at　the　lower　flame　front　were　observed　in　all　tests,　with　the　oscillation　frequency　increasing　with　pv.　Acoustic　oscillations　with　a　1200-Hz　frequency　occur　when　pv≥12　kPa.　The　maximum　internal　overpressure　increases　with　the　distance　to　the　vent.　The　maximum　internal　overpressures　near　the　vent　and　at　the　duct　center　increase　almost　linearly　with　pv.　However,　the　maximum　internal　overpressure　at　the　duct　bottom　is　not　increasing　monotonically　with　pv.　The　pressure　peak　resulting　from　the　external　explosion,　which　increases　with　pv,　always　dominates　the　external　pressure–time　histories.　In　addition,　external　explosion　affects　the　venting　process　in　all　tests,　but　the　effect　is　significantly　weakened　when　pv　≥　31　kPa.　©　2022　Explosion　and　Shock　Waves.　All　rights　reserved.