The polycrystalline samples of magnesium (Mg)-modifiedPb(Nb3/4Mo1/4)O-3 (PNM) (i.e., (Pb1-zMgz)(0.375)(Nb0.75Mo(0.25))O-3, where z=0.0, 0.03, 0.05, 0.07) were synthesized by a high-temperature solid-solution reaction technique. Some aspects of crystal structure of the compounds at room temperature were studied using an X-ray diffraction (XRD) technique. Studies of dielectric properties (dielectric constant (epsilon) and tangent loss (tan delta)) both as a function of frequency (100 Hz to 100 kHz) and temperatures (290-450 K) exhibit dielectric anomaly in the range of 365-391 K suggesting a possible ferroelectric-paraelectric phase transition in the compounds. The transition temperature increases on increasing Mg content in PNM. The activation energy (E-a) calculated from the plot of ac conductivity versus inverse of absolute temperature was found to be 0. 15 ev. (c) 2007 Elsevier BN. All rights reserved.