The thermal expansion coefficient and physical aging behavior of cured epoxy resins modified with amino-terminated polydimethylsiloxane were investigated. The epoxy resin was tetramethyl biphenol diglycidyl ether. Two factors influenced the thermal expansion behavior and the free volume relaxation of the polysiloxane-modified epoxy systems. One was the miscibility between the epoxy resin and the polysiloxane and the flexibility of the chains in the network system. The intrinsic thermal expansion of the network chains and the volume relaxation increased as a result of building polysiloxane molecules into the network structure. The other factor was the size and concentration of the dispersed polysiloxane particles. The increased local free volume at the interface between the epoxy matrix and the polysiloxane particle resulted in a higher thermal expansion coefficient and led to a large driving force for the volume relaxation during annealing. (C) 1998 John Wiley & Sons, Inc.