In the trend to decrease design features in microelectronics, porous SiOCH are good candidates to meet the request for low dielectric constant materials. This paper deals with the determination and the evolution of the different contributions to the dielectric constant (electronic, ionic and dipolar). These contributions are evaluated using the temperature dependence of the permittivity and refractive index measurements. Physicochemical and electrical characterizations are performed in order to enlighten the impact of porosity increase and post-treatment on the different contributions to the dielectric constant. Our study shows that a continuous decrease of the ionic fraction is observed when the porosity increases, which is mainly linked to the Si-O-Si concentration evolution. The dipolar contribution is related to the presence of porogen residues and Si-O-C bonds. Finally, our results are compared to literature, putting in evidence the existence of a lower limit for the dielectric constant of porous SiOCH thin films. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.jes113605] All rights reserved.