A small-angle neutron scattering (SANS) investigation of matrix chain deformation in an especially designed composite model system fora filled rubber-elastic network is presented. SANS experiments on the structure of polyisoprene homopolymer chains introduced into a well-microphase-separated triblock polyisoprene-polystyrene-polyisoprene (PI-PS-PI) system of the ABA type are reported. Use is made of the phase- and composition-matching techniques. The data are described in terms of the scattering behavior of the corresponding pure polyisoprene network at larger microscopic than macroscopic strains. The results confirm the model for the strain enhancement in reinforced networks and give the first direct microscopic insights into the mechanisms of reinforcement. A complementary small-angle X-ray scattering (SAXS) study yields all relevant geometrical parameters of the model filler and its detailed behavior under strain.