The mechanical properties of a hollow glass microsphere-filled epoxy resin composite system were investigated in air as a function of the weight content of particles ranging from 0 to 15.38% (wt/wt). The analysis based. on necessarily modified classical models together with structural observations enabled explanation of the composition dependencies of elastic moduli of the blends studied. It was found that, together with a presumption of no interfacial adhesion between the filler and matrix, an interlayer of immobilized matrix with changed mechanical properties compared to the formal matrix seems to strongly affect the mechanical behavior of the composite.