An area 1360 km2 in extent on the eastern side of the South Kenya rift valley consists of block faulted Plio-Pleistocene flood lavas ranging from basalts to trachytes. The oldest flood lavas are the Limuru trachytes (1.91 Ma BP) which were followed by the Ol Keju Nero basalts. A second eruptive cycle began with the Ol Tepesi basalts (1.42–1.65 Ma), which contain a very thick porphyritic benmoreite flow, and were followed by the Plateau trachytes (0.8–1.25 Ma). The magnetic polarities of the lavas agree with new K–Ar dates, and allow a revised and expanded stratigraphic succession to be established. The tectonic evolution of the eastern margin of the South Kenya rift valley followed pre-rift eruption of peralkaline lavas from shield volcanoes (13.4–3.3 Ma), and consisted of formation of the eastern boundary fault (3.3–2.3 Ma), formation of an inner graben (1.9–1.65 Ma), succeeded by almost continuous block fault movements (0.8–0.4 Ma). It is postulated that two major basalt-trachyte eruptive cycles occurred during late Pleistocene evolution of the southern rift valley, and that the voluminous flood trachytes could have been formed by fractional crystallization from a large basic intrusive body emplaced under the axial zone of the rift.