Ultrasonic model experiments were performed to examine the effect of a transition layer on the frequency spectra of refraction arrivals. Two-dimensional, layered structures with transition layers constructed from polystyrene and Plexiglas sheets served as the models. The materials were so chosen that the model simulated the relation between the crust and the mantle. A pulsed sinusoidal wave with variable frequency and a square-wave pulse were used. The amplitude of the refraction arrivals showed a rapid decrease with increasing frequency starting at a frequency f0 which was found to have the value 1.7 k, where k is the velocity gradient in the transition layer. It is expected that a similar effect will be observed for the refraction arrivals from the Mohorovičić discontinuity if a transition layer exists at the boundary.