Mapping, K–Ar dating and palaeomagnetic polarity measurements were used to study the geological history of the predominantly volcanic sequences in the western part of the Kenya rift valley S of 1º30'S. The rift was initiated as a downwarp which was later infilled by melanephelinitic agglomerates and lavas (15–12 Ma ago). Major faulting (12–7 Ma ago) downthrew these volcanics to the E and flood trachytes (7–5 Ma ago) were erupted onto the newly-formed rift floor. Minor faulting and volcanism preceded the second period of major faulting (5–3 Ma ago). Extensive flood basalts (3–2.5 Ma ago) infilled the resulting rift structure. Later subsidiary depressions in the rift floor were infiIled by two large porphyritic trachyte flows in the N (the older one 2.1 Ma ago), by flood basalts (2.1–1.7 Ma ago) in the S and by flood trachytes (1.4–0.7 Ma ago) in the E. The palaeomagnetic polarity sequence from the rift floor lavas represents a substantial part of the Matuyama epoch. A period of major faulting (most of it post-0.6 Ma ago) created much of the present rift topography and the resulting Magadi–Natron basin was later flooded by an extensive temporary lake.