A silsesquioxane compound mixture having a high proportion of silsesquioxane compounds bearing bulky substituent groups on side chain and having a degree of condensation of substantially 100% is prepared by a first stage wherein a silane feed comprising a trifunctional silane bearing a bulky substituent group on side chain represented by formula (1): ##STR00001## wherein Y is an aliphatic or aromatic organic group optionally having a functional group, X.sup.1, X.sup.2 and X.sup.3 are H, halogen, alkoxy or aryloxy is hydrolyzed in the presence of an acid or base catalyst, and a second stage wherein dehydrating condensation is carried out in the presence of a strong base catalyst while removing the water resulting from condensation out of the reaction system.