In　this　study,　experiments　on　the　vented　deflagration　of　stoichiometric　hydrogen-methane-air　mixtures　were　performed　in　a　300　mm　(length)　x　300　mm　(width)　x　1000　mm　(height)　duct　with　a　250　mm　x　250　mm　vent　on　the　top　surface,　to　investigate　the　effect　of　hydrogen　fraction　in　fuel　(chi)　ranging　from　0　to　1.0　on　flame　evolution　and　pressure　buildup　within　and　outside　the　duct.　The　results　reveal　that　chi　significantly　affects　flame　propagation　and　pressureetime　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　chi　in　current　tests.　For　a　given　chi,　the　maximum　overpressure　in　the　duct　increased　with　the　distance　to　the　vent.　Both　the　maximum　internal　and　external　overpressure　increased　slowly　as　chi　increased　from　0　to　0.7,　and　increased　quickly　with　a　further　increase　in　chi.　Violent　external　explosions　occurred　for　chi　=　0.8　and　1.0,　and　an　extremely　high　internal　overpressure　appeared　under　the　effect　of　the　external　explosion.　(C)　2020　Hydrogen　Energy　Publications　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.