The sol-gel transition of iota carrageenan and pectin is followed by measuring the evolution of ultrasonic velocity of a compressional wave at a frequency of 500 kHz. Measurements are performed as a function of temperature, from about 90 to 20 degreesC. Results are compared to those obtained in rheology in oscillatory conditions. It is shown that the ultrasonic velocity is sensitive to the sol-gel transition when going from a viscous to a "solid-like" state with a large elastic component. However, the method fails when the transition occurs gradually to give a weak elastic gel. (C) 2002 Elsevier Science Ltd. All rights reserved.