Aiming at Pt nanoparticle catalysts for application in polymer electrolyte membrane fuel cell (PEMFC) cathodes, which are highly active and more corrosion resistant under realistic mobile applications than common Pt/C catalysts, we have prepared and investigated Pt catalysts supported on highly stable, nanostructured composite materials containing carbon nanotubes (CNTs) and titania. TiO2@CNT composite materials are synthesized via sol-gel processing and subsequent Pt deposition. The physical and electrochemical properties as well as the stability of these catalysts, as determined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), rotating ring disk electrode (RRDE) measurements and accelerated degradation tests (ADTs), were compared with those of commercial Pt/C, Pt/TiO2 and Pt/CNT. The measurements reveal a high activity of the composite catalyst, comparable to that of the Pt/C catalyst, but an almost complete loss of ORR activity upon an ADT procedure simulating start-stop behavior. In contrast to carbon supported catalysts, where degradation is mainly associated with corrosion at high potentials, we find the titania supported catalysts to mainly suffer from the reductive treatment in the ADTs. Consequences for the use of Pt catalysts supported on reducible oxides such as TiO2 as cathode catalysts in fuel cell applications are discussed.