Raman spectroscopic studies of the model network derived from poly(propylene oxide) with tris(4-isocyanatophenyl) thiophosphate (triisocyanate) in the presence of salt, LiClO4, have been carried out over a range of salt concentrations and molecular weights of poly(propylene oxide). The spectral analysis in the frequency region of 800 and 300-500 cm(-1) representing characteristic vibrational modes of poly(propylene oxide) chains showed the salt to interact preferentially with the cross-link points, i.e. the urethane groups rather than the ether groups of the poly(propylene oxide) chain. Also, the relative intensity changes in the 1700 cm(-1) carbonyl stretching region with increasing salt concentration provided a direct indication of the salt interaction with the urethane groups. Previously, an impedance spectroscopic study suggested that there are two ionic conduction mechanisms related to the ether units and the urethane groups, respectively. Our spectroscopic studies proved this hypothesis. Therefore ionic conductivity in this particular network-based polymer electrolyte is determined by the combination of two mechanisms, one coupled to the poly(propylene oxide) chains and the other to the urethane groups present at the cross-link points.