The rubbery epoxy network, based on diglycidyl ether of Bisphenol A (DGEBA) and poly(oxypropylene)diamine (Jeffamine D2000), was reinforced with a nanometer-sized inorganic building blocks-polyhedral oligomeric silsesquioxanes (POSS). The organic-inorganic networks contained POSS as pendant units of a network chain or as network cross-links of various functionality. Thermomechanical properties and thermal stability of the POSS-containing networks were determined by DMA and TGA. The strongest reinforcement was achieved in the networks with pendant POSS forming ordered crystalline domains, which act as physical cross-links. The POSS skeleton with "soft" flexible substituents, such as octyl, shows formation of weak aggregates only, which do not contribute to reinforcement. The rubbery modulus of the networks with POSS in a junction grows with increasing POSS functionality due to enhanced network cross-link density. These networks are more homogeneous, and the modulus of the network with the octafunctional POSS junction well agrees with theoretical prediction. The mechanical properties are affected mainly by POSS-POSS interactions while the POSS-network chain interactions are of minor importance.