We estimated the irreversibility lines from the resistive-superconducting transition in DC magnetic fields up to 1 T for the Bi1.6Pb0.4Sr2Ca3Cu4O12 superconducting samples prepared by liquid ammonium nitrate with the same heat treatment but 25 h and 200 h of annealing time. Increasing the annealing time from 25 h to 200 h at 840 degrees C shifted the irreversibility temperature to a higher temperature. A non-linear Arrhenius plot of the resistive tail has been found. The activation energy obtainable from Arrhenius plots seems to be non-linear temperature dependent. The peak temperature T-p, in the imaginary part of the AC susceptibility seems to be shifted to a higher temperature by nearly the same amount as the shift in irreversibility temperature with increasing annealing time. Temperature variation of the critical current density extracted from the theoretical calculation of matrix susceptibility and irreversibility line obey the same power law dependence of (1 - T/T-c)(n), where n = 2.7 and 2.5 for the 25-h and 200-h samples, respectively. (C) 2000 Elsevier Science S.A. All rights reserved.