Abstract.We have performed precision elastic backscattering measurements for the weakly bound nuclei 6,7 Li on the medium and heavy mass targets 58 Ni, 116,120 Sn and 208 Pb at sub-and near-barrier energies (E c.m./Vbar = 0.6 to 1.3). Excitation functions of elastic scattering cross-sections have been measured at ±160• and ±170• and the corresponding ratios to Rutherford scattering and relevant barrier distributions have been extracted. These measurements complement recent work on a 28 Si target, for probing systematically the potential at sub-and near-barrier energies and relevant reaction mechanisms, while indicate a possible breakdown of the dispersion relation.