The northernmost spreading centre of the world, the Gakkel Ridge, is also an end-member in terms of global spreading velocities. Models show that full spreading rates vary between 1.3 and 0.63 mm yr-1 along the almost 1800 km long ridge system in the Central Arctic Ocean. The western part of the ridge was investigated in great detail by a two-ship expedition in summer 2001. The complete data sets and the modelling of the seismic refraction and aeromagnetic experiments gathered during this expedition are shown in this study. The magnetic signals along the dense (2 km spacing) aeromagnetic flight lines acquired at the same time show a good correlation between high amplitudes and a shallowing of the rift valley and the presence of large volcanic constructions at the rift shoulders. The magnetic anomalies rapidly fade out east and west of these centres of focused magmatism. This might indicate that the basaltic layer producing the magnetic anomaly thins away from the volcanic centres. A continuous magnetic anomaly is observed along the rift valley west of 3°30′E, consistent with increasing and more robust magmatism. The crustal thickness along the Gakkel Ridge varies greatly. Beneath some of the centres of focused magmatism, the oceanic crust thickens up to 3.5 km. In the amagmatic segments in between the crust thins to 1.4-2.9 km. This observation is also valid for the Western Volcanic Zone west of 3°30′E, where despite the stronger magnetic anomaly the crust does not significantly thicken. The strength of the magnetic anomaly along the rift valley is thus not a reliable indicator of crustal thickness beneath the Gakkel Ridge. The data show that the crustal thickness does not change dramatically across 3°30′E. Only the occurrence of a large elongate volcanic ridge significantly influences this parameter. More frequent volcanic eruptions along such ridges are most likely responsible for the basalts found in the westernmost part of the Gakkel Ridge. In the non-transform segments some seismic stations indicate that mantle rocks are exposed at the seafloor, with no indication of the presence of a basaltic cover or normal oceanic crust. Both the seismic and magnetic data support models in which the uppermost basaltic cover is responsible for the magnetic anomaly in the rift valley. © 2007 The Authors Journal compilation © 2007 RAS.