The ultrasonic properties of three oil field cement slurries are studied during the early stages of the hydration process. As a percolating solid framework is established, the slurries develop mechanical integrity and shear waves begin to propagate. This transition is also apparent in the behavior of the compressional wave, but is considerably more clear-cut in the shear signals. For the three samples examined here, the ratios of the shear wave onset times are in good agreement with the corresponding ratios of the American Petroleum Institute (API) thickening times.