The surface characteristics of ultrathin La0.75Sr0.25MnO3 (LSMO) films epitaxially grown on SrTiO3 (0 0 1) substrate were studied as a function of film thickness. The X-ray photoelectron spectroscopy (XPS) analysis displays that the ratio of Mn3+/Mn4+ in LSMO largely decreases with decreasing film thickness, which reduces the amount of itinerant electrons for electrical conduction. The conductive atomic force microscopy images also reveal that the fraction of conductive phase drastically decreases with decreasing film thickness to 5 nm, ice. an insulative dead layer is formed. From the above observations, it is believed that, besides the strain effect, the formation of dead layer at the interface may also be related to the large reduction of Mn3+ ions in the LSMO adjacent to the SrTiO3 substrate. (C) 2008 Elsevier B.V. All rights reserved.