Angular distributions of ^6,7Li scattered by ²⁷Al have been measured at different bombarding energies between 6 and 18 MeV. The results obtained from an optical model analysis using several potentials indicate that there is an isotopic dependence in the energy variation of the real and imaginary strengths. Whereas the ⁷Li + ²⁷Al system shows no indication of any threshold anomaly, the ⁶Li + ²⁷Al reaction suggests the presence of the so-called breakup threshold anomaly.