We report acoustic laboratory measurements on initially unconsolidated sand packs with hydrate formation taking place in the pore space. Both P and S wave velocities were measured. Hydrate was formed from water and the refrigerant R11 (CCl3F) at 2 ¿C and atmospheric pressure. Measurements were performed on two types of initially unconsolidated sand packs with average grain size 100 μm and 280 μm, respectively. P wave velocities varied from about 1700 m/s at low hydrate content to 3810 m/s at a calculated hydrate fraction in the pore space equal to 0.52. S wave velocities were only detectable at hydrate fractions larger than 0.35, where S wave velocities up to 2230 m/s were measured. Our measurements indicate that acoustic measurements are not sensitive to small amounts of hydrate in a system of initially unconsolidated sand. The data show a very distinct change in behavior when the hydrate fraction exceeds 0.35, the concentration at which hydrate cementation of the sand grains is believed to occur. Measurements for samples without sand (samples containing water, R11, and hydrate) are also reported. For hydrate without sand, P wave velocities ranged from about 1400 m/s at very low hydrate content to about 2500 m/s at a measured hydrate fraction equal to 0.68. For these samples, reliable S wave data were not detected. ¿ 1999 American Geophysical Union