Thiol-functionalized nanobuilding blocks (NBBs) were synthesized from 3-mercaptopropyltrimethoxysilane by using the in situ water production (ISWP) process in which the water needed to hydrolyze the precursor was provided by means of an esterification reaction. In the present study the reaction between 1-propanol and chloroacetic acid was used. Whereas the growth of the Si oligomers was followed at room temperature and 100 °C using 1D 29Si and 2D 1H–29Si heteronuclear single quantum correlation (HSQC) NMR spectroscopic experiments, the amount of water delivered along the process was followed by means of 1H NMR spectroscopy. The results show a good correlation between the evolution of the degree of condensation and the amount of water produced in situ. They also point to the preferential formation of cagelike structures and the narrowing of the species distribution over long reaction times. The average size of the growing oligomers was estimated from their diffusion coefficient, which was measured by means of 1H diffusion-ordered NMR spectroscopy (DOSY NMR). Like gel permeation chromatography, DOSY NNR showed a plateau between 70 to 100 hours in the growth of the oligomers, a time at which, according to 29Si NMR spectroscopy, the well-defined octakis(3-mercaptopropylsilsesquioxane) is the major species.