Owing to the insolubility of the binary phase system for oxidative desulfurization, a formic acid/H2O2 system with quaternary ammonium salts as phase-transfer catalysts was employed in the oxidation of thiophene. Four catalytic systems with ultrasound were carried out, tetrabutyl ammonium bromide behaved as the optimum active catalyst, and the desulfurization rate was 94.67%. Dispersion of phase-transfer catalyst between the organic and aqueous phases was related to the extraction constant. When the amount of catalyst exceeded 0.0019 mol L-1, quaternary ammonium salts would serve well to transfer the polar substance of oxidant [HCOOO-] to the nonpolar environment of the organic phase. In the transfer process, complexation [HCOOO-Q-X] resulted from the interaction of oxidant and phase-transfer catalyst, which could decrease the polarity of the oxidant and the apparent activation energy. With the extractive equilibrium and oxidative reaction, a dynamic model was developed. From the study of kinetics, it could be shown that the reaction order was pseudo-first-order.