Low-k SiOC(-H) films with different structural orders were fabricated by using UV-assisted PECVD, and the influence of the replacement of Si-O bonds to the (Si-R) bonds on structural and electro-physical properties has been investigated. By analyzing the changes of the C-V characteristics, we found that the excess -CH 3 or Si-CH 3 and Si-H bonds in SiOC(-H) films produced the effective positive charges coming from excessive interstitial defects. UV irradiation depopulates these defect states at the range of rf-power 400-600 W. Observed shifts in the flat-band voltage (V fb) toward zero volts with increasing RF power indicate that positive trapped charges can be neutralized by effective negative charges coming from excessive oxygen. It was found also that the space-charge-limited current mechanism dominant at rf-power below 700 W. At higher RF power and under UV irradiation, this is followed by Poole-Frenkel emission mechanism. © 2011 American Institute of Physics.