During field surveys between 2005 and 2011 in the Tugen Hills by the Franco-Kenyan Kenya Palaeontology Expedition, several hominid specimens were discovered in the Pliocene Mabaget Formation. One mandible fragment, three isolated teeth and a pedal phalanx collected from the Pelion Member (base of the formation aged 5.0-4.5 Ma) are compatible in dimensions with Orrorin tugenensis and Ardipithecus ramidus whilst a mandible from the Sinibo Member, a younger level in the formation (ca. 3.4-3.0 Ma) represents an appreciably larger species, as big as, or bigger than, Praeanthropus afarensis (ex-Australopithecus afarensis) from locality AL 333, Ethiopia. The small hominid mandible and an isolated p/3 were found in the type section of the Mabaget Formation at localities 2/211 and 2/210 respectively, in deposits aged ca. 5.0-4.5 Ma. An isolated upper milk molar, a lower third molar and a pedal phalanx are from Sagatia, near Rondinin, also aged between 5.0 and 4.5 Ma. The large mandible was collected at Sinibo, near Kipcherere, from sediments above the local occurrence of the Tulu Bor Tuff (= Sidi Hakoma Tuff) which is dated at 3.446 Ma. The aim of this paper is to describe and interpret these hominid fossils and to place them within their geological, stratigraphic and palaeoenvironmental contexts.