The inner demagnetization factor Ni of polymer-bonded soft magnetic composites (PBSMC) varying in filler material and filler content C was measured, using a computer-controlled measurement system. For all materials the results show a decrease from an inner demagnetization factor close to the demagnetization factor of a single particle to NiE0 on increasing the magnetic content C. Measurement data of a selected material were compared with a model derived from an effective medium theory using the composite's effective susceptibility and show superb accordance.