The　change　in　pH　during　excess　sludge　(ES)　fermentation　of　varying　sludge　concentrations　was　investigated　in　a　series　of　reactors　at　alkaline,　acidic,　and　neutral　pHs.　The　results　showed　that　the　changes　were　significantly　affected　by　fermentative　conditions.　Under　different　conditions,　pH　exhibited　changing　profiles.　When　ES　was　fermented　under　alkaline　conditions,　pH　decreased　in　a　range　of　(10　+/-　1).　At　the　beginning　of　alkaline　fermentation,　pH　dropped　significantly,　at　intervals　of　4　h,　4　h,　and　5　h　with　sludge　concentrations　of　8665.6　mg/L,　6498.8　mg/L,　and　4332.5　mg/L,　then　it　would　become　moderate.　However,　under　acidic　conditions,　pH　increased　from　4　to　5.　Finally,　under　neutral　conditions　pH　exhibited　a　decrease　then　an　increase　throughout　entire　fermentation　process.　Further　study　showed　short-chain　fatty　acids　(SCFAs),　ammonia　nitrogen　and　cations　contributed　to　pH　change　under　various　fermentation　conditions.　This　study　presents　a　novel　strategy　based　on　pH　change　to　predict　whether　SCFAs　reach　their　stable　stage.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.