The crustal structure of the transition from the NE Iberian Peninsula to the western Mediterranean is characterized by large-scale variations related to its successive compressional and extensional Cenozoic tectonics. The Spanish ESCI (Spanish acronym for seismic studies of the Iberian crust) Program recorded 450 km of deep vertical reflection profiles on land and at sea, to complete a first crustal transect from the Pyrenees to the South Balearic basin. Highly energetic, north dipping reflections imaged by combining the Etude Continentale et Oc¿anique par Reflexion et Refraction Sismique (ECORS) and ESCI land profiles mark a crustal thickening toward the Pyrenees. The reflective band becomes horizontal beneath the Catalan coastal ranges, and the Moho is found at 11s. No evidence of crustal thinning affecting the Iberian mainland is observed. The transition to the Mediterranean basin is constrained by complementary onshore-offshore recordings. Forward modeling combined with multichannel processing of the wide-angle data set shows that a strong thinning of the continental crust by a factor of 2 over a distance of 50 km affects both the Catalan and Balearic flanks of the Valencia trough. This thinning affects mainly the deep crust, characterized by low velocities of 6.4--6.5 km/s, suggesting a removal of part of the lower crust during the rifting processes. Velocities of 7.8--7.9 km/s are interpreted in the uppermost mantle with no evidence of underplating. The Moho appears as a marked discontinuity throughout the transect. The NW part of the Balearic promontory is characterized by a thick and reflective crust from 12 to 30 km depth. An intensely fracturated evaporitic sequence is seen for 100 km along the south Balearic basin. The seismic images of the continent-ocean transition are disturbed by these prominent sediments. However, the deep reflectivity suggests a crustal thinning (Moho at 6--7 s) to be related to the Middle Miocene-present extensional tectonics. ¿ 1998 American Geophysical Union