The aim of this study was to observe the effect of ultrasound and sonication on whey proteins in order to improve their functional properties. Effect of ultrasound treatment on physicochemical and functional properties was examined by pH, conductivity and solubility measurements and foaming properties. In this work, low-intensity ultrasound (500 kHz) and the high-intensity ultrasound (20 kHz probe and 40 kHz bath) were used. 10 wt.% protein model suspensions of whey protein isolate (WPI); whey protein concentrate (WPC-60); and whey protein hydrolysate (HWP) were treated with ultrasound probe (20 kHz for 15 and 30 min) and ultrasound baths (40 kHz and 500 kHz for 15 and 30 min). pH did not change significantly upon ultrasound treatments. Ultrasound affected functional properties (using 20 kHz probe) of whey proteins like solubility and foaming ability by sample exposure at high temperatures caused by sonication. Using ultrasound of 40 kHz frequency had less effect on protein properties and better results were obtained with 15 min treatment than with 30 min treatment. Ultrasound treatment with 500 kHz bath did not had effect on foaming properties of whey protein model solutions. Conductivity decreased for ultrasound treatments with 40 kHz and 500 kHz bath for all samples. Temperature of protein model solutions increased after all ultrasound treatments. (C) 2007 Elsevier Ltd. All rights reserved.