Ultralow dielectric constant pSiCOH films have been prepared using allyltrimethylsilane(ATMS) as the skeleton precursor and a porogen precursor. The porogen has been removed from the deposited films by thermal annealing at 420 °C, obtaining films with dielectric constants down to 2.2. The films have been investigated by Fourier transform infrared spectroscopy, and n&k optical measurements of the refractive index (n) and the electrical characteristics have been measured on metal-insulator-semiconductor structures. It was found that the properties of the annealed films depend on the deposition temperature and concentration of porogen.