The esterification of sodium salicylate to synthesize n-butyl salicylate was investigated via ultrasound-assisted tri-liquid phase-transfer catalysis. The third-liquid phase containing the catalytic intermediate tetra-n-butylphosphonium salicylate (ArCOOQ) was prepared from the reaction of sodium salicylate and tetra-n-butylphosphonium bromide with a large excess of NaBr in the aqueous phase. Under ultrasonic irradiation at 28 kHz/300W, the product yield in the organic phase was 97.4% at 70 degrees C after 100 min of reaction in a batch reactor. The reaction rate increased with decreasing ultrasonic frequency in the range of 28-80 kHz. The variations of ArCOOQ in the third-liquid phase during reaction were analyzed. A pseudo-first-order kinetic model was proposed to describe the overall reaction. The increment in apparent rate constant was 53.3% under ultrasonic irradiation. The assistance of ultrasound in third-liquid phase-transfer catalysis provides an efficient and environmentally benign method to synthesize esters. (C) 2009 Elsevier B.V. All rights reserved.