Phase transfer catalysis is used widely in the synthesis of various organic chemicals in both liquid-liquid and solid-liquid systems. The C-alkylation of benzyl cyanide by 2-bromopropane in the presence of sodium hydroxide was investigated. The reaction rate of the phase transfer reaction can be increased by ultrasound, but frequency and power of ultrasound and the geometry of the ultrasonic field inside the reactor influence the reaction strongly. The results demonstrate clearly, that to achieve a high efficiency of ultrasonic systems in chemical processes the ultrasonic parameters and geometry of the sonotrode had to be adopted.