The ultrasonic properties (at 2.25 MHz) of lactose solutions and suspension of lactose crystals (d similar to 50 mu m) were measured as a function of concentration (0-40 wt.%). Ultrasonic velocity increased linearly with concentration regardless of the state of dissolution of the lactose crystals while ultrasonic attenuation was low and concentration-independent when the lactose was dissolved and increased approximately linearly with the concentration of suspended crystals. Therefore the amount of lactose present and the state of dissolution can be determined simultaneously with single ultrasonic sensor. A sensor based on this principle was applied to a stirred tank and used to measure the time taken to mix powdered lactose into a solution and the time for the added lactose to dissolve. (C) 2009 Elsevier Ltd. All rights reserved.