In　this　study,　experiments　on　the　vented　deflagration　of　stoichiometric　hydrogen–methane–air　mixtures　were　performed　in　a　300　mm　(length)　×　300　mm　(width)　×　1000　mm　(height)　duct　with　a　250　mm　×　250　mm　vent　on　the　top　surface,　to　investigate　the　effect　of　hydrogen　fraction　in　fuel　(χ)　ranging　from　0　to　1.0　on　flame　evolution　and　pressure　buildup　within　and　outside　the　duct.　The　results　reveal　that　χ　significantly　affects　flame　propagation　and　pressure–time　histories.　The　movement　of　the　entire　flame　bubble　toward　and　away　from　the　vent　and　a　spike-shaped　structure,　which　had　been　observed　in　a　vented　methane–air　mixture　in　a　previous　study,　was　observed　for　low　χ　in　current　tests.　For　a　given　χ,　the　maximum　overpressure　in　the　duct　increased　with　the　distance　to　the　vent.　Both　the　maximum　internal　and　external　overpressure　increased　slowly　as　χ　increased　from　0　to　0.7,　and　increased　quickly　with　a　further　increase　in　χ.　Violent　external　explosions　occurred　for　χ　=　0.8　and　1.0,　and　an　extremely　high　internal　overpressure　appeared　under　the　effect　of　the　external　explosion.　©　2020　Hydrogen　Energy　Publications　LLC