Dielectric behavior of alternating ethylene-carbon monoxide (ECO) and ethylene-carbon monoxide/propylene-carbon monoxide elastic terpolymer (EPEC-48) (48% mole ratio of ECO) have been investigated by impedance spectroscopy. At intermediate frequencies, the co- and terpolymer samples display a relaxation peak with an activation energy of 1.40 and 1.12 eV, respectively, due to a rotational motion of the carbonyl groups in the relatively more polar ECO block. However, at low frequency, a relaxation peak was observed only in the EPEC-48 terpolymer impedance and admittance spectra due to the rotational motion of the polar groups of propylene-carbon monoxide (PCO) block which is attached to ECO repeating units. In addition, it has been found that below 100 degrees C the ECO material is highly resistive, and both dielectric constant and loss factor are dependent on frequency and temperature. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2020-2026, 2012