Electrical conductivity and dielectric properties of lithium manganese spinels, Li1+dMn2-dO4, d=O and d=0.005, obtained by sol-gel method, were studied by impedance spectroscopy. Parameters of relaxation function were obtained by fitting model response to the measured impedance. As evidenced by the X-ray powder diffraction, samples heat-treated at 800 degrees C undergo upon cooling a phase transition from cubic to an orthorhombic structure at about 16 degrees C. Appearance of the orthorhombic distortion is accompanied by a decrease of conductivity by a factor about 10. In the orthorhombic phase, two distinct relaxation processes were observed: (i) at high frequencies, a dielectric like process exhibiting relaxation strength Delta epsilon(D) congruent to 5 and an activation energy about 0.25 eV; (ii) at frequencies near the onset of the conductivity dispersion, a charge carrier relaxation. Samples obtained by sol-gel synthesis and heat-treated up to 580 degrees C do not undergo a phase transition and exhibit only charge carrier relaxation in their dielectric function. The activation energy of the charge carrier relaxation has nearly the same value as that of the dc conductivity, i.e. about 0.33 eV The charge carrier relaxation strength is between Delta epsilon(C) congruent to 11 for samples sintered at 800 degrees C and Delta epsilon(C) congruent to 80 for samples calcined at 300 degrees C. (c) 2005 Elsevier B.V. All rights reserved.