The mechanism of silsesquioxane growth is studied with an insertion mechanism analogous to the one previously proposed for fullerene growth. Different from previous work, the possibility of the involvement of larger rings with (SiO)7 size in cages of hydridosilsesquioxanes Si n O3 n /2H n is investigated. Such a nonclassical pathway is calculated for the growth of Si30O45H30 via Si32O48H32 to Si34O51H34. The MSINDO method is used to determine the transition structures and intermediates. It is found that nonclassical pathways are equally preferable as classical pathways involving cages with (SiO)5 and (SiO)6 rings only.