The lateral surface diffusion at Si-SiO2 interface has been observed at nanometer scale using photoelectron emission microscopy (PEEM) combined with synchrotron soft X-ray excitation. The samples investigated were Si-SiOx micro-patterns prepared by O-2(+) ion implantation in Si (0 0 1) wafer using a mask. The lateral spacial resolution of the PEEM system was about 41 nm. The brightness of each spot in the PEEM images changed depending on the photon energy around the Si K-edge, in proportion to the X-ray absorption intensity of the corresponding valence states. It was found that the lateral diffusion occurs by 400-450 degrees C lower temperature than that reported for the longitudinal diffusion at the Si-SiO2 interface. It was also found that no intermediate valence states such as SiO (Si2+) exist at the Si-SiO2 interface during the diffusion. The observed differences between lateral and longitudinal diffusion are interpreted by the sublimated property of silicon monoxide (SiO). (C) 2011 Elsevier B.V. All rights reserved.