Experiments　on　the　effects　of　ignition　position　and　hydrogen　ratio　(chi)　on　the　explosions　of　CH4/H-2/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　chi　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　p(2)　with　amplitude　sensitive　to　both　ignition　position　and　chi.　The　effects　of　ignition　position　and　chi　on　the　build-up　of　the　internal　pressure　were　not　significant　when　chi　＜=　0.15.　The　maximum　explosion　overpressure　in　the　vessel　(p(max))　under　the　explosions　of　rear　and　central　ignitions　(RI　and　CI)　increased　monotonically　as　chi　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　＜=chi　＜=　1.0　except　for　chi　=　0.7,　because　FI　resulted　in　the　highest　p(max)　at　chi　=　0.7.　A　pressure　peak　outside　the　vent　(p(ext))　caused　by　the　combustion　expansion　of　the　combustible　cloud　could　be　distinguished　when　chi　＞=　0.45　in　the　explosions　of　RI　and　CI.　The　amplitude　of　p(ext)　increased　with　an　increase　in　chi.　Rear　ignition　always　led　to　the　highest　p(ext)　when　chi　＞　0.6.