The backscattering cross-sections for 6 Li and 7 Li ions scattered from natural carbon have been measured in the energy range 1.7-5.5 MeV at a laboratory scattering angle of 165 o . The threshold energies above which Rutherford backscattering becomes invalid are 3700 keV for 6 Li and 3520 keV for 7 Li. Above these thresholds the cross-sections decrease below Rutherford, while the background due to nuclear reactions increases strongly. The measured threshold energies are compared to theoretical predictions. The cross-section data are presented in tabular and graphical form.