Thermal and dielectric studies were investigated on hybrid composite of Alfa fibers reinforced polyester matrix. Taking into account the presence of wool fibers in the non-woven Alfa fibers reinforcement, these studies revealed the effect of wool fibers on the thermal stability of the reinforcement and hence on the hydrophilic character of the composite. Indeed, Thermo Gravimetric Analysis (TGA) revealed an enhancement of the moisture regain of the reinforcement, attributed either to the cystine structure of wool fibers or to its hygroscopic character. Contact angle method has shown a medium hydrophilic character of the composite. Differential Scanning Calorimetery (DSC) showed a plasticizer effect of the reinforcement explained by chemical bonds based on secondary bonding between the fibers and the matrix. Dielectric measurements confirmed such thermal analyses by exhibiting the presence of the dielectric relaxations at low temperatures. Furthermore, the analysis of dielectric relaxations at high temperatures illustrated Debye-type behavior of the interfacial polarization effect attributed to a homogenous distribution of the reinforcement in the polyester (PE) matrix. (C) 2015 Elsevier B.V. All rights reserved.