p-C5H5NC16H33](3)[PW4O16] was reported to be an excellent epoxidation catalyst which exhibited a unique reaction-controlled phase transfer behavior. In the paper, the composition and structural changes of the reaction-controlled phase transfer catalyst during and after reaction have been investigated by P-31 NMR spectroscopy. The P-31 MAS NMR confirmed that the original catalyst was a mixture of heteropoly tungstophosphates. When the catalyst reacted with hydrogen peroxide, the species {PO4[WO(O-2)(2)](4)}(3-), [(PO4){WO(O-2)(2)}(2){WO(O-2)(2)(H2O)}](3-) and [(PO3(OH)){WO(O-2)(2)}(2)](2-) were detected by in situ P-31 NMR. It was also found that the P/W ratios and quaternary ammonium cations had great influence on the composition of heteropoly tungstophosphates. Although the catalyst with [(C18H37)(2)N(CH3)(2)](+) was not a reaction-controlled phase transfer catalyst, it could be precipitated from the reaction solution when acetone was subsequently added to the solution. The P-31 MAS NMR spectra of the recovered catalysts revealed that they had more low P/W ratio heteropoly tungstophosphates than fresh catalysts.