Formation, reactions and functionalization of completely and incompletely condensed oligosilsesquioxane are discussed. To systematically understand the mechanism of formation of oligosilsesquioxane, reaction conditions to obtain polysilsesquioxanes, especially oligosilsesquioxanes with controlled structure, mainly by hydrolysis of tri-functional phenylsilane, are described paying attention to the possible reaction mechanism of the formation of completely and incompletely condensed oligomeric silsesquioxanes. Special emphasis was given to the importance of the cyclic intermediate in the reaction. (c) 2007 Elsevier Ltd. All rights reserved.