The Porcupine Basin west of Ireland formed by extension during the Jurassic and earliest Cretaceous. The basin opens out to the south and the amount of subsidence increases in the same direction, indicating that the amount of extension increases toward the south from low values typical of the North Sea rift in the north to those observed at rifted margins in the south. Indeed, the crust thins to at most a few km, and may be absent in the south. We study the structure and tectonic evolution of the basin by using a series of seismic profiles (reflection and wide-angle) across the basin to determine crustal structure at a variety of latitudes. The resulting cross-sections across the basin provide the crustal structure and structural geometry at a variety of stages in the evolution of a rifted margin, from a continental rift in the north to two conjugate margins separated by a mini ocean basin in the south (Reston et al., 2004). The seismic reflection data reveal a detachment fault cutting down from the east flank of the data to the west, and possibly across the crust-mantle boundary (Reston et al., 2001). Wide-angle data collected in May 2004 will test this interpretation. Further south, the reflection profiles reveal a lens-shaped high on top of the tilted fault blocks. Although previously interpreted as basaltic, we suggest that this may consist of mobilised serpentinites (Reston et al., 2004). In a model for the development of rifted margins by progressive extension, we suggest that faults cut through the thinning crust and into the mantle, allowing water to reach and serpentinise the peridotites (Perez-Gussinye and Reston, 2001). Serpentinites are very weak (allowing the development of detachment faulting such as P) and low density, and so can rise diapirically to the surface. To constrain the structure further, we have collected a series of wide-angle profiles using OBH/OBS across the basin during Meteor cruise M61-2. Equipment used consisted of two types of ocean bottom seismometers, ocean bottom hydrophones, and 2-3 32 litre airguns. A British marine gravimeter (thanks to Dr. C. Peirce) provides complementary recording of the gravity data. Data quality is excellent and data analysis promises to determine the structure of the Porcupine Basin and strongly constrain the mechanics of formation of rifted margins.