Two-ship multichannel seismic profiles, using expanding spread and constant offset source-receiver configurations, were shot with explosive charges and a 16.4 l air gun to investigate the structure of the continental shelf off northwest Britain. A long-range (69 km) expanding spread profile reveals that the crystalline basement off northern Scotland is covered by sedimentary sections up to 2.5 km thick, and is divisible into two seismic units with velocities of 6.1 km/s and 6.6 km/s. Prominent supercritical Moho reflections indicate a crustal thickness of 26.7 km. Strong sedimentary and basement refractions, together with oblique reflections from the vicinity of the Moho, have been profiled at a constant air gun-receiver offset of 10 km across a sedimentary basin west of the Orkney Islands.On the Outer Hebridean shelf to the west of mainland Scotland, the metamorphic basement lies within 250 m of the sea floor. P- to S-wave conversion occurs at the basement surface; V p /V s gives a Poisson's ratio of 0.27-0.31 at depths of 300-1 000 m. Marked changes in mode conversion efficiency are observed on constant offset profiles and are attributed to variations in the velocity structure of an uppermost low-velocity (4.9 km/s) layer of weathered basement. The deeper crustal velocity appears uniform with depth, although there is some evidence of significant lateral velocity changes (6.28-6.61 km/s). In contrast to the shelf north of Scotland, reflections from within the basement and from near the base of the crust are recorded only sporadically on constant-offset profiles. A strong event at 10.4 s two-way reflection time appears to have arisen from a seismic discontinuity within the upper mantle. The differences in seismic character of the basement on the two-ship profiles suggest significant variations in crustal structure within the Caledonian foreland of northern Britain.