Experiments　on　the　effects　of　ignition　position　and　hydrogen　ratio　(χ)　on　the　explosions　of　CH4/H2/air　mixtures　were　experimentally　investigated　in　a　vented　cylindrical　vessel.　Relevant　experiments　were　conducted　utilizing　ignition　sources　situated　at　the　rear　(rear　ignition),　the　center　of　the　vessel　(central　ignition),　or　near　the　vent　(front　ignition)　with　χ　varying　from　0　to　1.0.　Two　types　of　cellular　structures　on　the　flame　surface,　owing　to　the　diffusional-thermal　instability　and　acoustically　enhanced　combustion,　respectively,　were　observed,　and　the　latter　resulted　in　acoustic　oscillations　of　the　overpressure　within　the　vessel　and　a　unique　overpressure　peak　p2　with　amplitude　sensitive　to　both　ignition　position　and　χ.　The　effects　of　ignition　position　and　χ　on　the　build-up　of　the　internal　pressure　were　not　significant　when　χ　≤　0.15.　The　maximum　explosion　overpressure　in　the　vessel　(pmax)　under　the　explosions　of　rear　and　central　ignitions　(RI　and　CI)　increased　monotonically　as　χ　was　increased　from　0.3　to　1.0,　but　a　nonmonotonic　trend　was　found　in　the　explosions　of　front　ignition　(FI).　CI　could　be　regarded　as　the　worst-case　scenario　when　0.45≤χ　≤　1.0　except　for　χ　=　0.7,　because　FI　resulted　in　the　highest　pmax　at　χ　=　0.7.　A　pressure　peak　outside　the　vent　(pext)　caused　by　the　combustion　expansion　of　the　combustible　cloud　could　be　distinguished　when　χ　≥　0.45　in　the　explosions　of　RI　and　CI.　The　amplitude　of　pext　increased　with　an　increase　in　χ.　Rear　ignition　always　led　to　the　highest　pext　when　χ　＞　0.6.　©　2024　Elsevier　Ltd