Structure, morphology and thermomechanical behaviour of organic-inorganic (O-I) networks prepared from O-I precursors were studied. The networks from two types of the precursors were formed (a) by the hydrolytic polycondensation of the alkoxysilane-endcapped oligomers (ASO) and (b) from epoxy-functional silsesquioxane (SSQO) clusters (EC) by reaction with an oligomeric diamine, JEFFAMINE D2000. The O-I block copolymer networks are composed of regularly arranged SSQO domains forming junctions of the network linked by oligomer chains. Structure of the SSQO 'crosslinks' ranges from compact polyhedral cages of size 1-3 nm in the networks from ASO to 515 nm spherical SSQO clusters in the stoichiometric networks EC-D2000. Large interconnected SSQO cylinders 30-150 nm in diameter arranged in a continuous ladder-like structure are formed at deficiency of the amine. Bicontinuous morphology results in a significant reinforcement. Interphase H-bond interaction between SiOH of SSQO domains and the organic chain leads to the chain immobilization and affects glass transition temperature and mechanical properties. (C) 2004 Published by Elsevier Ltd.