Rubbery crosslinked epoxide was reinforced with silica-siloxane structures formed in situ by sol-gel process from tetraethoxysilane. The increase in modulus by two orders of magnitudes was achieved at a low silica content (<10 vol%). Various polymerization procedures including simultaneous or sequential formation of the epoxide network and silica resulted in different structures of the microphase separated organic-inorganic hybrid composite. Structure and morphology of the heterogeneous system were analyzed by electron microscopy and small-angle X-ray scattering. Temperature dependences of storage modulus and loss factor were used to evaluate phase structure of the hybrids and the interaction between epoxy and silica phases. Efficiency of the reinforcement depends on the reaction conditions; a crucial effect of an interphase formation is shown. Acid catalysis of the sol-gel process probably promotes grafting between epoxide and silica phases and leads to a more uniform and finer structure with smaller silica domains. Comparison of the mechanical data with the theoretical models reveals that studied composites correspond to the morphological model of the hybrid consisting of co-continuous epoxy and silica structures.