In this paper, the dielectric response of lanthanum-modified Pb(Fe1/2Ta1/2)O-3 relaxor ferroelectrics is studied. La ions are intended to replace Pb ions (A-site) of perovskite structural block where the electrical neutrality is maintained via the generation of negatively charged B-site vacancies. XRD examination shows that predominantly single-phase can be achieved using the one-step preparation approach throughout the entire range of doping. A peak dielectric permittivity maximum (epsilon(max)') at slight doping (0.3 mole% La2O3) is observed, origin of which is discussed. Addition of La2O3 lowers the Curie temperature (T-max) whereas the frequency-dispersion of epsilon(max)' is found to be hardly affected implying the ferroelectric couplings among oxygen octahedra being nearly unaltered. The paraelectric to ferroelectric phase transition, however, can be rendered increasingly broader by La-doping, among which the 0.3 mole% La2O3 addition exhibits the most prominent effect. (C) 2002 Elsevier Science B.V All rights reserved.