New dielectric films are prepared by both pyrolytic and photolytic conversion of beta-chloroethyl-silsesquioxane (BCESSQ). Film thickness, refractive index, composition, density and morphology are characterized using a palette of techniques including ellipsometry, Rutherford backscattering and forward recoil spectrometry, X-ray reflectivity, and atomic force and electron microscopies. After annealing at 350 degrees C, BCESSQ films, initially 200 nm thick reach about 55% of their original thickness after 20 min. For films heated in air for 4 h, the atom fractions of carbon and hydrogen monotonically decrease to 10 and 30%, respectively, as annealing temperature increases from 225 to 450 degrees C. The BCESSQ reactivity is reflected in the loss of chlorine at 400 degrees C. At 450 degrees C, the film density is 1.88 g/cm(3), or 84% of thermally-grown silicon oxide. Upon exposure to ultraviolet ozone radiation, films ranging from ca. 200 to 700 nm are found to convert to ormosil films within 30 min. Surprisingly, the chlorine concentration is found to decrease more quickly than the hydrogen and carbon concentrations, suggesting that the ormosil him evolves HCl leaving a vinyl group of Si-CH=CH2. This reaction pathway differs from the thermal case. For films prepared by pyrolytic and photolytic methods, atomic force and electron microscopy studies show that the surface is smooth and featureless, the bulk is void free when view at a magnification of 50 000 x, and the ormosil/substrate interface is continuous.