In this paper we present for the first time the positron lifetime data for Al-substituted Y1 : 2 : 3 superconductors. The formula composition of experimental samples can be expressed in YBa 2Cu 3−x,AI.OY with x = 0 ∼ 0.4. The positron annihilation characteristics and Al-substitution dependence of the lifetime parameters is given. Anomalous changes in these parameters were found near x = 0.15, which corresponds to the known structural transition from an orthorhombic to a tetragonal phase. The results prove the presence of electronic weak localization with increasing Al-substitution. The effect of the electronic weak localization on charge transfer and on superconductivity are also discussed.