Measurements of the ac conductivity and the dielectric loss of Pb modified Ge1-XSe2Pbx disordered system (x = 0, 0.2 and 0.6) have been carried out in the frequency range 100Hz-20kHz and at temperature from 303 to 433 K. The virgin and the x = 0.2 samples data shows a normal behavior according to the relation alpha(ac)(omega )= Aomega(s). However, the x = 0.6 sample behaves differently. The exponent(s) measured for the two modified samples shows sub-linear relation depending on the temperature of measurements. The experimental results of the highly modified sample are interpreted in terms of the coexistence of two parallel conduction mechanisms; the predominance of each depends on both temperature and frequency. (C) 2003 Elsevier B.V. All rights reserved.