New mixed valence oxides KTi 2−xNb 5+xO 17', chemical twins of the rutile structure have been isolated, with O ⩽ x ⩽ 1.75. The electron transport and magnetic properties have been studied and compared to those of reduced rutile. These physical properties are interpreted in terms of a band model and of the existence of Nb(IV)-Nb(IV) pairs and Ti(III) species.