Polyurethane systems corresponding to different reaction extents of the gelation process are investigated by ultrasonic methods in order to apprehend their high-frequency dynamical properties and their evolution during formation. The longitudinal moduli M calculated from the measurement of the longitudinal velocity and absorption exhibit a relaxation behavior which is attributed to the alpha process. The results are analyzed by comparison with low-frequency rheological data, allowing a more complete description of this relaxation by the Tamman-Vogel-Hesse-Fulcher equation. As moduli M are obtained versus temperature at a single frequency, data are treated by a transformed Havriliak-Negami function. The alpha relaxation process probed by ultrasonics is larger than the rheological one, which has been interpreted in terms of the influence of a secondary relaxation merging at high temperature.