SUMMARY A unified two‐dimensional lithospheric model has been derived for the P‐wave velocity structure beneath the Alpine Longitudinal Profile (ALP'75) main line by making new phase correlations and using ray‐tracing and ray‐theoretical seismogram calculations in laterally inhomogeneous media. Due essentially to an increase in thickness of the lower crust, the total crustal thickness increases from 30–40 km in the peripheral regions to about 50 km in the central region of the Alps. The thickness change is more abrupt at the eastern end than at the western end of the line. The boundary between the upper and lower crust occurs at 18–20 km depth except in the peripheral region at the eastern end of the line where it occurs at 14–16 km depth. Along the whole line the seismic basement has velocities averaged over some tens of kilometres of 5.85‐6.15 km s‐1 with individual values having uncertainties of ±0.05‐0.1 km s‐1 at least east of shot‐point B. The lower part of the upper crust has been modelled as a low velocity layer, which is most pronounced beneath the peripheral regions, and which is generally underlain by a transition zone at the base of which the top of the lower crust occurs. The top of the lower crust has been modelled with velocities ranging from 6.25‐6.5 km s‐1 beneath the peripheral regions to 6.5‐6.65 km s‐1 beneath the central Alps. Velocity uncertainties at the top of the lower crust are estimated to be around ±0.2 km s‐1. Beneath the central Alps a low velocity layer has also been modelled in the lower crust. The top 6 km of the mantle has been modelled with velocities of 8.0–8.2 km s‐1 with individual estimates having uncertainties of ±0.15 km s‐1. Below this, the upper mantle down to almost 100 km depth has been modelled as a series of high velocity layers embedded in material with lower velocities of 8.0–8.2 km s‐1. The combined effects of tectonic overthrusting and uplift and erosion have probably led to the lower crust being apparently thicker than the upper crust in the central Alps at the present day.