As part of the geophysical investigations into the uplift of the Rhenish Massif, a long-range seismic refraction experiment with areal coverage was carried out during 1978–1979 to determine the velocity structure of the crust and uppermost mantle beneath the massif (Mechie et al., 1983). The experiment included a 600 km long main profile extending from the Paris Basin in the southwest, across the massif itself, to the Hessian Depression in the northeast, and three cross profiles, up to 170 km long, almost completely located in the massif itself. Interpretation of the data, including ray-tracing through laterally inhomogeneous media and calculation of synthetic seismograms, has yielded the following velocity-depth information. The P g phase represents the refracted wave, with velocity 6.0–6.4 km/sec, from the upper crust below an average depth of 3–5 km. Other intracrustal phases, of variable lateral extent and occurring in the depth range 9–24 km, can also be recognized. The crust-mantle boundary structure beneath the profiles also displays marked lateral variability. Between shot-points A and B, beneath the Paris Basin, there exists a sharp crust-mantle boundary at 32 km depth. Along the main profile, between shot-points B and D, beneath the Southern Ardennes, an upper mantle velocity of 8.1 km/sec is reached at about 37 km depth, at the base of a 7.5 km thick transition zone. Northeast of shot-point D, beneath the massif itself, there is a thin (1 km) high-velocity layer ( V = 8.1 km/ sec) beneath a sharp crust-mantle boundary at 29–31 km depth. Below the thin high-velocity layer, there is a 4–6 km thick transition zone in which the velocity increases from low values (6.3–7.0 km/sec) to upper mantle values (8.4 km/sec). Along the cross profiles, mainly towards the northwest of the main profile, it appears that the crust-mantle boundary structure is a 2–4 km thick transition zone, with an upper mantle velocity of 8.1 km/sec being reached at 30–33 km depth. An attempt is made to explain the results obtained from the seismic experiment in terms of a petrological model. In particular, the transition zone from low velocities (6.3–7.0 km/sec) to high velocities (8.4 km/sec) in the depth range 30–35 km along the main profile northeast of shot-point D may be explained in terms of magmatic material rising from greater depths and being trapped at the level of the crust mantle boundary structure.