X-ray photoelectron spectroscopy (XPS) has been used in the study of sol gel-derived Er3+-activated xHfO2–(100−x)SiO2 (x=10, 20, 30, 40, 50mol) planar waveguides. The analysis of Si 2p and O 1s core lines were related to the Hf/Si molar ratio to assess the role of hafnia in modifying the silica network. Increasing the HfO2 content brings about a change of the Si 2p and O 1s binding energy respect to those from pure silica. This trend is explained with a formation of hafnium silicate in the matrix with successive phase separation between HfO2 and SiO2 rich phases. XPS results show that hafnia is well dispersed in the silica matrix for molar concentration below 30%. Formation of pure HfO2 domains was detected at higher hafnia concentrations in agreement with previous spectroscopic analyses.