Synchrotron radiation photoelectron spectroscopy (SRPES) has been applied to surface analysis of indium tin oxide (ITO) thin films. Several different components of In and Sn were observed at the clean ITO surface. By comparing the chemical compositions of the film before and after vacuum annealing, the contents of In2O3-x and Sn3O4 were found to be the major factors influencing the electrical conductivity and optical transparency of the film.