AbstractIn this work tannin was isolated from the outer bark of the eucalyptus tree, then treated with Ba(OH)2 to synthesize barium tannate (Ba-tan). The derivative was evaluated as a thermal stabilizer for polyvinyl chloride (PVC). PVC was mixed with Ba-tan thermally at four percentages (0.5, 1.0, 2.0, and 2.5)% w/w. The samples were pressed thermally to films having thickness ranging between 0.25 and 0.30 mm. The effect of Ba-tan was studied by monitoring the weight of samples under iso-thermal conditions at 250 oC. Thermal stability of PVC increased with increasing Ba-tan percent. Samples weight curves also indicated that the additive had been suitably active to increase the resistance of PVC against thermal degradation, where the degradation rates decreased from about 0.21 % wt/min. at 0.5% of Ba-tan on PVC to reach to 0.05% wt/min. for the 2.5% treated sample. The ability of Ba-tan to react with and remove HCl from the system of polymer during thermal conditioning of samples and during the measurement led to this effect. Therefore the present results demonstrate the performance of this derivative as a thermal stabilizer.