PVC stabilised with mixtures of pre-heated metal stearates are degraded thermally for specific periods of time, in order to determine their influence on the production of polyenes by uv spectroscopy. Additionally, the PVC samples are treated with NaBH4 to reduce the carbonylic groups produced during the thermo-oxidative process. Important differences in the polyene formation trends are observed, between the reduced and nonreduced samples, indicating that the carbonyl groups attached to polyenes shift the absorbance maxims and alter considerably the quantification of the double bond conjugations.