Processible poly(methylsilsesquioxane)s (PMSSQs) were prepared in refluxing THF solutions under nitrogen atmosphere in the presence of HCl catalyst. It was found that various reaction parameters such as concentration, temperature, reaction time, relative amount of water, and relative amount of acid catalyst could affect the molecular weight, microstructure, and the amount of functional end-groups of synthesized PMSSQs. PMSSQ thin films prepared with high molecular weight PMSSQ samples synthesized in solutions exhibited a much improved crack resistance over commercially available samples, probably due to the effects of different microstructures of polymers. The dielectric constants of the fully cured thin films prepared in this study were found to be ca. 2.7, which is nearly the same as those for commercially available samples.