The width of metastable zone of unseeded potassium nitrate, potassium sulphate and ammonium sulphate solutions, saturated at 30 degreesC, was determined by means of an ultrasonic measuring technique at three different cooling rates. The change of ultrasonic velocity and temperature in solutions during a cooling and heating cycle was controlled by using a protected sensor. The results were compared with those of the unprotected sensor taken from the literature for the chosen substances. The change of ultrasonic velocity of supersaturated ammonium sulphate solutions in the presence of crystals was measured at constant temperature by means of a protected sensor. It was found that the protected sensor was reliable and sensitive enough to determine the change of ultrasonic velocity and temperature in both solution and slurry. (C) 2003 Elsevier Science B.V. All rights reserved.