A new method has been developed that uses model u.v./visible absorption spectra to determine the relative resonance enhancement factors and the ranges of polyene lengths that contribute to a particular resonance Raman spectrum from a thermally degraded poly(urethane) foam-backed poly(vinyl chloride) sheet. Resonance Raman spectra were recorded using Microline Focus instrumentation with 514.5, 488.0, 457.0 and 632.8 nm excitation. The individual polyene nu(2) resonance Raman bands contributing to the observed nu(2) resonance Raman band have been determined and polyene concentration profiles have been calculated. Interpretation of the data shows that polyene propagation is the dominant mechanism in the degradation. Detailed modelling of the 514.5 nm excitation data, over the full range of degradation times, indicates an inverse relationship between concentration and polyene length. This new result suggests that there is no maximum length to which a polyene propagates in this system.