The evolution of conductivity in tetragonal tungsten bronze (TTB) oxides belonging to the Pb5(1-x)Li10xTa10O30 solid solution was studied using two techniques: a.c. complex impedance spectroscopy and the four-probe method. The results obtained from both techniques allowed the correlation of the evolution of conductivity with composition. Indeed, the conductivity mainly appears to be governed by the total number of Li+ ions in the structure. Furthermore, it seems that when the number of Li+ ions in the structure is small, two separated regions appear: a low temperature region (LT), characterized by activation energy Delta E-LT, a very small number of mobile ions and a low value of the prefactor sigma(0); a high temperature region (HT), characterized by activation energy Delta E-HT (Delta E-HT > Delta E-LT), a larger number of charge carriers and a higher value of the prefactor sigma(0). The existence of these two regions is explained in terms of extrinsic-intrinsic behavior. Dielectric measurements were also performed, with the substitution 2Li(+)-Pb2+ leading to a decrease of the Curie temperature which is explained in terms of chemical bonding and antagonist effects. (C) 2000 Elsevier Science B.V. All rights reserved.