High resolution X-ray Photoelectron Spectroscopy (XPS) study of K2O-SiO2 glasses: Evidence for three types of O and at least two types of Si

► Evidence for free oxygen (O2−) in potassium silicate glasses. ► Possible evidence for two types of bridging oxygen. ► Si 2p XPS spectra resolve Q4 and Q3 species consistent with NMR studies. ► BO:NBO abundances change as function of exposure time to X-ray beam. ► More O2− in K-silicate glass than in Na-silicate glass at equivalent alkali content.