The　characteristics　of　nitrite　accumulation　during　denitrification　in　mixed　liquors　with　different　pH　values　were　studied.　The　mixed　liquors　were　prepared　by　adding　acclimated　denitrifying　activated　sludge　to　synthetic　wastewater　with　methanol　(CH　3OH　)　as　the　carbon　source　(chemical　oxygen　demand=600　mg/L　)　and　sodium　nitrate　(NaNO　3)　as　the　nitrogen　source　().　The　results　showed　that　the　pH　during　denitrification　could　be　kept　constant　using　buffer　solutions　composed　of　KH　2PO　4,　Na　2HPO　4.12H　2O　and　Na　2B　4O　7.10H　2O.　Nitrite　accumulation　was　more　serious　at　low　pH　than　at　high　pH,　regardless　of　whether　the　pH　of　the　mixed　liquor　was　stabilized　during　denitrification.　The　specific　rates　of　nitrate　and　nitrite　reduction　were　both　inhibited　by　increased　pH.　The　specific　rate　of　nitrate　reduction　was　more　vulnerable　than　that　of　nitrite　reduction　to　higher　pH.　Nitrite　accumulation　during　these　experiments　resulted　from　different　reduction　rates　for　nitrate　and　nitrite.　Nitrite　reduction　rates　were　inhibited　by　the　presence　of　nitrate,　which　stimulated　nitrite　accumulation　during　denitrification.