In this work, we undertook a comparative study of the dynamic dielectric analysis of two composites: natural-fiber-reinforced unsaturated polyester (NFRUP) and E-glass-mat-reinforced unsaturated polyester (EGMRUP). In both composites, two common relaxation processes were identified, the first of which was the -mode relaxation associated with the glass transition of the matrix. The second one was associated with conductivity that occurred because of the carriers' charge diffusion and was observed at temperatures above the glass transition and at low frequencies. However, the interfacial or MaxwellWagnerSillars polarization was noticed only in the NFRUP composite. This dielectric study also revealed that compared to E-glass fibers, natural fibers enhanced the thermal insulation in the composite. Also, the study of the fiber adhesion in the matrix with scanning electron microscopy, differential scanning calorimetry, and tensile testing revealed a great compatibility of the fibers with the matrix in both composites. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013