Frequency and temperature dependence of dielectric constant (epsilon(r)), dielectric loss (tan delta), ac conductivity (sigma(ac)) and complex impedance spectroscopy studies on cured polyester matrix and sisal fibre-reinforced polyester composites (SFRPC) have been investigated in the frequency range from 180 Hz to 1 MHz and temperature range from room temperature to 200 A degrees C. The experimental results showed that with the incorporation of sisal fibre, the values of epsilon(r), tan delta and sigma(ac) are increased. It is also found that the values of epsilon(r) and tan delta for both cured polyester matrix and SFRPC are decreased with increasing frequency, which indicates that the major contribution to the polarization may come from orientation polarization and interfacial polarization. The increasing value of epsilon(r) with increasing temperature at a particular frequency is due to free motion of the dipole molecular chains within the cured polyester matrix and SFRPC at higher temperature.