The complex ac susceptibility of polycrystalline Bi1.6Pb0.4Sr2Ca2Cu3O10+delta pellet as a function of temperature, ac field amplitude, frequency and dc field is reported. Data have been analyzed using a non-linear flux diffusion model, where thermally assisted vortices diffuses over an effective pinning barrier. Using the temperature corresponding to the peak in chi", the dependence of the effective barrier potential U(T, H-d, J) on temperature, dc field and current density has been determined and the results are found to be consistent with those reported earlier. The critical current density (J(C)) as a function of dc field is also estimated using the extracted barrier and compared with those obtained from I-V characteristics at 77 K. This agreement between the calculated and measured values indicates that estimation of J(C) of HTSCs using magnetic methods (viz. ac susceptibility) should be based on frequency-dependent non-linear models. (C) 2002 Elsevier Science Ltd. All rights reserved.