High-molar-mass silsesquioxanes (SSQO) based on (3-glycidoxypropyl)trimethoxysilane (GPMS) and (3-methacryloxypropyl)trimethoxysilane (MPMS) were synthesized. The hydrolytic condensation of GPMS was performed using HCOOH (0.1 N) as catalyst, keeping the molar ratio H2O/Si = 3. A first step was performed in tetrahydrofuran (THF) at 50 degrees C, followed by a second step in diglycidyl ether of bisphenol A (DGEBA), where temperature was increased in steps up to 140 degrees C. The hydrolytic condensation of MPMS was performed in bulk with HCOOH 98%, at T = 50 or 70 degrees C, using molar ratios of HCOOH/Si = 3 or 6. Homogeneous solutions were obtained for both silanes. The reaction was followed by size exclusion chromatography (SEC), and final products were characterized by matrix-assisted ultraviolet laser desorption/ionization time-of-flight mass spectrometry (UV-MALDI-TOF MS). FTIR, and H-1 and Si-29 NMR. Molar-mass distributions showed the presence of clusters corresponding to products formed in different generations. With the aid of UV-MALDI-TOF MS, the different: species present in every cluster could be identified for one of the silsesquioxanes derived from MPMS. During the initial stage of the hydrolytic condensation, species with 7-12 Si atoms were produced. They mainly consisted of incompletely condensed polyhedra (species with 1-3 OH per molecule) and ladder-type structures (species with 4 OH per molecule). Species with more OH groups were condensed with a higher probability. giving place to a second generation of products. This process accounts for the presence of a cluster of species with 14-24 Si atoms and the enrichment of the first cluster in the more condensed structures (T-7(OH), T-8(OH)(2), and T-9(OH)). Third and fourth generations of condensation products were also present. Structures of different species may be depicted as combinations of incompletely condensed polyhedra with ladder fragments.