The Kenya Rift International Seismic Project (KRISP) seismic refraction-wideangle reflection experiments carried out between 1985 and 1994 show abrupt changes in Moho depths and Pn phase velocities as the rift boundaries are crossed. Beneath the rift flanks, normal Pn phase velocities of 8.0-8.3 km s-1 are observed, except for the Chyulu Hills volcanic field, east of the rift, where it is 7.9-8.0 km s-1. Also to the east, some of the thickest crust (38-44 km) encountered so far beneath Kenya has been observed over a distance of c. 300 km. However, beneath the surface expression of the rift itself, the uppermost mantle velocity of the Pn phase is anomalously low at 7.5-7.8 km s-1 throughout ist length. Beneath the rift itself, there are major differences in crustal thickness, extension and upper mantle velocity structure between the north and the south. Beneath the section from the centre of the Kenya Dome southwards, where the extension is estimated to be 5-10 km, the crust is thinned by c. 10 km to a thickness of 35 km, and the narrow low-velocity zone in the mantle extends to a depth of at least 65 km. However, in the north beneath Turkana, where the extension is 35-40 km, the crust is only c. 20 km thick and two layers with velocities of 8.1 and 8.3 km s-1 are embedded in the low velocity mantle material at depths of 40-45 km and 60-65 km. this mantle velocity structure indicates that the depth to the onset of melting is at least 65 km beneath the northern part of the rift and is thus not shallower than the corresponding depth (45-50 km) in the south. These results, taken together with those from teleseismic studies, petrology and surface geology, have been used to deduce that anomalously hot mantle material appeared below the present site of the Kenya Rift c. 20-30 Ma ago. This led to widespread volcanism along the whole length of the rift and modification of the underlying crust by mafic igneous underplating and intrusion.