Dense silicon dioxide (SiO2) and nanoporous dielectric silicon oxycarbide (SiOCH3) films were investigated in aqueous solutions using electrochemical impedance spectroscopy (EIS). The influence of the film thickness of the dielectric layer was studied through the investigation of the electrochemical impedance characteristics of 120 and 330 nm thick films. Long-time effects of immersing the dielectrics in water were investigated by studying the changes in the Bode spectra over time. The dense but rather rough SiO2 films show no significant changes, indicating unchanged physical properties of the material with neglectable water absorption. However, EIS measurements evidenced that the porous SiOCH3 matrix readily adsorbs water. A pure capacitive behavior was observed at short immersion times, while a second time constant with constant phase element behavior was recorded for the 120 nm thick SiOCH3 matrix after 5 h and for the 330 nm film after 20 h immersion in the aqueous solution. These observations are consistent with moisture absorption. (c) 2007 The Electrochemical Society.