The longitudinal sound velocities in crosslinked polyacrylate hydrogels were measured as a function of water content in the gel in the temperature range 30-100 degrees C. The sound velocity changed drastically around a water content of 40-50 wt%. Above this range, the sound velocity decreased monotonically and approached that of pure water. The maximum sound velocity was observed around 74 degrees C for a water content above 50 wt%, while for a water content less than ca. 40 wt% the temperature dependence curve of the sound velocity became concave. The amount of water bound to the gel was estimated and was discovered to vary depending on the ratio of sodium polyacrylate to polyacrylic acid and the temperature.