Dielectric materials based on Si, C., O, H (SiCOH) have been demonstrated previously with dielectric constants of about 2.8. This value could be potentially further reduced by increasing/introducing porosity in the SiCOH films. Depositing multiphase films containing at least one thermally unstable phase and annealing the films to remove this labile phase from the material could create the enhanced porosity. Dual-phase materials, SiCOH-CH, have been prepared in the present study by PECVD from mixtures of a SiCOH precursor with a hydrocarbon. The films have been characterized as-deposited and after thermal anneals of up to 4 hours at 400°C. The atomic composition of the films has been determined by RBS and FRES analysis and their optical properties have been determined by FTIR and n&k measurements. Metal-insulator-silicon structures have been used to measure the electrical properties of the dual-phase films. After an initial anneal at 400°C, accompanied by a significant loss of CH and some SiH species and a thickness loss of up to 50%, the films stabilized. Depending on the deposition conditions and concentration of the CH precursor in the feed gas, the dielectric constant decreased by 10-15% during the stabilization anneal and reached values as low as 2.4. These initial results indicate the possibility to further reduce the dielectric constant of PECVD produced SiCOH films and the potential to incorporate such films in the interconnect structures of future ULSI chips.