Catalytic　epoxidation　of　alkenes　is　an　important　type　of　organic　reaction　in　chemical　industry,　and　the　deep　insight　into　catalyst　deactivation　will　help　to　develop　new　epoxidation　process.　In　this　work,　series　of　quaternary　ammoniums　bearing　different　cationic　sizes,　i.e.　MTOA+　(methyltrioctylammonium,　[(C8H17)3CH3N]+),　HTMA+(hexadecyltrimethylammonium,　[(C16H33)(CH3)3N]+)　and　DMDOA+　(dimethyldioctadecylammonium,　[(C18H37)2(CH3)2N]+)　were　incorporated　with　polyoxometalate　(POM)　anions　to　prepare　phase　transfer　catalysts　(PTCs),　which　were　used　in　the　styrene　epoxidations.　Among　them,　(MTOA)3PW4O24　exhibits　the　best　catalytic　performance　judged　from　the　highest　styrene　conversion　rate　(52%)　and　styrene　oxide　selectivity　(93%),　during　which　the　styrene　epoxidation　conditions　were　optimized.　Meanwhile,　the　deactivation　mechanism　of　this　kind　of　PTCs　was　proposed　firstly,　i.e.　in　the　case　of　low　H2O2　content,　the　oxidant　can　only　be　used　in　the　styrene　epoxidation,　in　which　the　catalyst　can　transform　into　stable　Keggin-type　POM.　But　when　the　content　of　H2O2　is　higher,　the　excess　H2O2　can　re-activate　the　Keggin-type　POM　into　active　(PW4O24)3−　anions,　which　can　trigger　the　ring-opening　polymerization　of　styrene　oxide.　Consequently,　the　catalyst　is　deactivated　by　adhered　poly(styrene　oxide)　irreversibly,　which　was　determined　by　NMR　spectra.　In　this　situation,　the　active　moiety　{PO4[WO(O2)2]4}3−　in　phase-transfer　catalytic　system　can　break　into　some　unidentified　species　with　low　W/P　ratio　with　the　presence　of　epoxides.　This　work　will　be　beneficial　for　the　design　of　new　PTCs　in　alkene　epoxidation　in　fine　chemical　industry.　©　2022　The　Chemical　Industry　and　Engineering　Society　of　China,　and　Chemical　Industry　Press　Co.,　Ltd