In　this　study,　experiments　were　performed　in　a　vented　cylindrical　vessel　to　investigate　the　effects　of　hydrogen　ratio　(chi)　on　the　vented　deflagration　of　hydrogen/methane/air　mixtures　with　an　ignition　source　located　near　the　vent　of　the　chamber　at　an　initial　pressure　of　101　kPa　and　initial　temperature　of　290　K.　Pressure-time　histories　in　the　vessel　were　obtained　via　a　piezoelectric　pressure　sensor,　and　flame　propagation　was　recorded　using　high-speed　photography.　The　experimental　results　revealed　that　the　flame　cellularity　appeared　earlier　as　chi　increased.　The　pressure　evolution　was　nearly　independent　of　chi,　as　it　was　no　higher　than　0.15,　but　depended　on　chi　in　the　tests　with　higher　values　of　chi.　Acoustic　oscillations　of　the　internal　overpressure　appeared　when　chi　ranged　from　0.3　to　0.9,　which　were　closely　related　to　the　fine　cells　formed　periodically　on　the　flame　surface.　The　pressure　peak　due　to　acoustically　enhanced　combustion　dominated　the　pressure-time　histories,　and　its　amplitude　first　increased　and　then　decreased　as　chi　increased　from　0.6　to　0.9.　The　overall　maximum　explosion　overpressure　(P-max)　did　not　increase　monotonically　with　an　increase　in　chi,　and　the　highest　value　was　reached　at　chi　=　0.7.　Therefore,　a　non-monotonic　trend　between　P(max　)and　laminar-burning　rate　of　hydrogen/methane/air　mixtures　was　obtained,　which　has　not　been　reported　in　previous　studies.