The effect of the proximity of Au nanoparticles on the transport and magnetic properties of ultrathin La 2/3 Sr 1/3 MnO 3 (LSMO) films has been investigated. We find a huge increase in the resistivity of the manganite (by four orders of magnitude for a Au nominal thickness of 2 nm), which is accompanied by a strong decrease in the Curie temperature. A combined scanning transmission electron microscopy and electron energy-loss spectroscopy analysis shows that interfaces are coherent and atomically sharp, and that the structural quality is very high. On the other hand, a strong reduction in the Mn oxidation state is seen upon Au capping. NMR data show a strong attenuation of the double exchange signal upon formation of Au nanoparticles. Ab initio calculations indicate a negligible influence of Au on LSMO at an ideal interface, with the LSMO surface magnetic and electronic properties essentially unchanged upon …