Polymer composites filled with mineral clays are becoming attractive because of their wide applications and low cost. In this study, polymeric composite produced from unsaturated polyester (UP) based on the glycolysis of poly (ethylene terephthalate) (PET) waste with diethylene glycol (DG) and untreated or treated kaolin was tested. Characterization of the unsaturated polyester was carried out using FT-IR and 1HNMR techniques. Thermal and chemical methods for treatment of kaolin were performed. The chemical treatment was carried out using different ammonium molybdate (dopant) concentrations to convert the alumina naturally present in kaolin from γ-alumina to α-alumina phase, which is a more stable one. These treatments affected positively for both mechanical and electrical properties of the polymer composites. The data obtained revealed that as the concentration of dopant used for chemical treatment of kaolin increased an enhancement in the properties of the composite is achieved, up to the value of 4mol%. Moreover the composite containing kaolin. The composite containing kaolin doped with 4mol% shows promising mechanical and dielectric properties which validate its use for insulating purposes. POLYM. COMPOS. 34:1223–1234, 2013. © 2013 Society of Plastics Engineers