This study used ultrasonic measurements to monitor the setting behaviour, and changes in the elastic modulus, of glass-ionomer cements. The ultrasonic equipment comprised a pulser-receiver, transducers, and an oscilloscope. The two-way transit time through the mixing cement disk was divided by two, in order to account for the down-and-back travel path, and then multiplied by the sonic velocity within the material. The sonic velocities of the longitudinal and shear waves were used to determine the elastic modulus. In the earliest stages of the setting process, most of the ultrasound energy was absorbed by the cements and the second echoes were relatively weak. As the cements hardened, the sound velocities increased until they reached a plateau. The changes in sound velocities differed among the glass-ionomer cements tested. The mean elastic moduli of the specimens ranged from 2.6 to 6.2 GPa after 15 min, from 13.4 to 20.4 GPa after 24 h, and from 11.4 to 22.4 GPa after 1 month. The ultrasonic method used in this study has considerable potential for determining the setting processes of luting cements.