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The liquid-phase hydroxylation of benzene into phenol by hydrogen peroxide with the participation of finely dispersed Fe / Zr oxide catalyst and the effect of ultrasound (US) on catalytic activity of synthesized systems have been analyzed. For this purpose, Hielscher ultrasonic device with frequency of 26 kHz and power of 200 W has been used.It found that the US effect within 10 minutes after each 1-hour work on the catalytic system that loses 80% of its activity within 4-6 hours results in restoration of its activity. It revealed that the US treatment of the system, along with its preservation in a dispersed state, cleans the surface of catalyst particles from oxidation products and leads to improvement of its performance properties in terms of high catalytic activity.The dependence of average diameter of particles in the catalytic system by the time of exposure to ultrasound has been analyzed. In the course of ultrasonic treatment of the catalytic system, there became apparent a smooth decrease in the mean diameter of the particles. As for the catalytic system which has been subjected to a 45-50 min treatment, the narrowest distribution of the catalyst particles in size is typical. It is believed that under the influence of ultrasound, the cavitation occurs in the liquid (emergence and explosion of numerous microscopic bubbles). When bubbles on the surface of the catalyst are exploded, energy is released to result incleaning the surface of the catalyst from reaction products. Using this way, the activity of the catalyst can be markedly intensified together with maintaining its long-term stable operation in theprocess. [ABSTRACT FROM AUTHOR]