The accuracy and precision of effective resonance energy (E) over bar (r) parameter is an important point in the determination of the mass fraction of an analyte contained in the sample when NAA is used. Because the effective resonance energy (E) over bar (r) is an essential parameter, which is directly introduced in the k(0)-NAA standardization method. Therefore, the effective resonance energies (E) over bar (r) for the Gd-158(n,gamma)Gd-159 and Hf-179(n,gamma)Hf-189m reactions were determined by cadmium ratio method using Mn-55 and Mo-98 monitors. The samples were irradiated in an isotropic neutron field of Am-241-Be neutron sources. The induced activities for the radioisotopes were produced in the samples with and without a 1 mm-thick Cd shield and then measured with a p-type high pure Ge detector. The correction factors required for thermal neutron self-shielding (G(th)) and resonance neutron self-shielding (G(epi)) for the irradiated samples, and self absorption (F-s) and true coincidence summing (F-coi) effects for the measured gamma-rays were taken into account. The experimental (E) over bar (r)-values are found to be 49.6 +/- 10.4 eV for Gd-158 and 17.0 +/- 3.5 eV for Hf-179 target isotopes, respectively. The (E) over bar (r)-values of Gd-158 and Hf-179 were also theoretically calculated by using the recent resonance data in the literature. Since the experimentally determined (E) over bar (r)-values for the Gd-158(n,gamma)Gd-159 and Hf-179(n,gamma)Hf-180m reactions are not available in literature, the present experimental results for the (E) over bar (r) energies are thus compared with the present and earlier theoretical values, respectively. The presently measured (E) over bar (r) value for Gd-158 is generally larger than those obtained (E) over bar (r) values from theoretical approaches by similar to 9.1-16.9%. Whereas, the theoretically calculated (E) over bar (r)-values for Hf-179 isotope deviated substantially from the presently measured value by similar to 16.9-37.6%. The reasons for this discrepancy are discussed in this paper. (C) 2011 Elsevier Ltd. All rights reserved.