In　this　study,　the　effects　of　vent　size　on　the　internal　overpressure　and　flame　behavior　within　and　outside　a　1　m3　cuboid　test　chamber　with　30　vol%　hydrogen–air　mixtures　were　investigated.　Different　pressure　peaks　and　the　corresponding　mechanisms　were　presented.　Two　regions　of　pressure　oscillation　were　found　in　the　pressure–time　histories.　The　oscillation　frequency　of　one　region　was　independent　of　the　nondimensional　vent　coefficient–　KV,　and　that　of　the　other　increased　considerably　with　KV.　The　maximum　overpressures　increased　slightly　when　the　vent　sizes　were　large　(KV　≤　12.50)　and　increased　significantly　as　the　vent　sizes　were　decreased　(KV　＞　12.50).　Two　empirical　correlations—Molkov＇s　best-fit　model　and　a　modified　version　of　NFPA　68　(2013)　by　Fakandu　et　al.—　relating　the　vent　sizes　to　overpressure　were　found　to　be　appropriate　for　the　present　study.　A　Mach　disk　was　identified　for　the　smallest　vent　size　(KV　=　25.00)　in　the　external　flame,　and　a　constant-pressure　jet　flame　was　observed.　©　2020　Elsevier　B.V.