The inhibitory effect of Al3+ on photosynthetic electron transport was investigated in isolated thylakoid membranes of spinach. A combination of oxygen evolution, chlorophyll fluorescence induction (FI) and decay and thermoluminescence measurements have been used to characterize photosystem II (PSII) electron transport in the presence of this toxic metal cation. Our results show that below 3 mm, Al3+ already caused a destabilization of the Mn4O5Ca cluster of the oxygen evolving complex (OEC). At these concentrations, an increase in the relative amplitude of the first phase (OJ) of FI curve and retardation of the fluorescence decay kinetics following excitation with a single turnover flash were also observed. A transmembrane structural modification of PSII polypeptides due to the interaction of Al3+ at the OEC is proposed to retard electron transfer between the quinones QA and QB. Above 3 mm, Al3+ strongly retarded fluorescence induction and significantly reduced Fv/Fm together with the maximal amplitude of chlorophyll fluorescence induced by a single turnover flash. This chlorophyll fluorescence quenching was attributed to the formation of P680+ due to inhibition of electron transfer between tyrosine 161 of D1 subunit and P680.