Methods of lining and/or filling gaps on a substrate by creating flowable silicon oxide-containing films are provided. The methods involve introducing vapor-phase silicon-containing precursor and oxidant reactants into a reaction chamber containing the substrate under conditions such that a condensed flowable film is formed on the substrate. The flowable film at least partially fills gaps on the substrates and is then converted into a silicon oxide film. In certain embodiments, the methods involve using a catalyst, e.g., a nucleophile or onium catalyst, in the formation of the film. The catalyst may be incorporated into one of the reactants and/or introduced as a separate reactant. Also provided are methods of converting the flowable film to a solid dielectric film. The methods of this invention may be used to line or fill high aspect ratio gaps, including gaps having aspect ratios ranging from 3:1 to 10:1.