We studied systems that are built up of a mixture of phenyltrimethoxysilane (PTMS) and (3-methacryloxy)propyltrimethoxysilane (MPTMS). Four different molar ratios on the monomers MPTMS/(MPTMS + PTMS) (0.1, 0.2, 0.3, and 0.5) were co-condensed in bulk using formic acid as catalyst (molar ratio HCOOH/monomers = 3) at 50 degrees C. The structures of the obtained silsesquioxanes were characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and electrospray ionization mass spectrometry (ESI-MS). Cohydrolysis of organoalkoxysilanes can promote cross-condensation as well as self-condensation. A new parameter, the condensation affinity between two organoalkoxysilanes (f(d)), is introduced in this study to discuss this effect. Our results indicate that the average amount of methacryloxypropyl groups in the final silsesquioxanes conserves the introduced proportion of MPTMS/(MPTMS + PTMS).