Despite their apparent simple preparation by sol-gel process, silsesquioxanes based materials are quite complex and rich systems. Accordingly, a clear understanding of their elaboration chemistry and its relations to process and properties are important matter. In the particular case of thin film preparation, vitrification must be controlled to ensure high throughput and reliability. Using methylsilsesquioxane based films as a relevant model and a combination of IR spectroscopy, mass spectrometry, and solution ²⁹Si NMR, a meaningful description of the film structure was obtained. Furthermore, vitrification, observed by dynamic mechanical analysis, was shown to depend strongly on the amounts of cages and extended structures present in the films. More interestingly, the proportion of these species can be tuned through a careful control of the elaboration chemistry. On one hand, long aging and high dilution yield a larger amount of cages, which results in a delay of the vitrification. On the other hand, pH appears as the convenient parameter to finely adjust the time span during which high quality films can be prepared from the sols, while preserving the film composition and final properties.