Abstract The electron transfer sites of p-benzoquinone ( pBQ) and 2,6-dichloro- p-benzoquinone (DCBQ) were investigated in thylakoid membranes and isolated photosystem II (PSII) particles from barley ( Hordeum vulgare) using α- and β-cyclodextrins (CD) at concentrations up to 16 mM. In CD-treated thylakoid membranes incubated with DCBQ the electron transport through PSII, estimated as oxygen evolution (OE), is largely enhanced according to a S-shaped (sigmoidal) dose–response curve displaying a sharp inflection point, or transition. The maxima percent OE enhancement at cyclodextrin concentrations above 14 mM are about 100% (α-CD) and 190% (β-CD). On the contrary, in thylakoid membrane preparations incubated with pBQ as electron acceptor one observes an OE inhibition of about 30% which might result from the depletion of the thylakoid membrane of its plastoquinone content. It was also found that in isolated PSII particles incubated with either pBQ or DCBQ the cyclodextrins induce only a small OE enhancement. Moreover, the observation in CD-treated thylakoid membranes incubated with pBQ of a residual, non-inhibited oxygen-evolving activity of about 70% puts a twofold question. That is, either the plastoquinone depletion was not complete, or, pBQ binds to electron acceptor sites of different nature. From this and data published in the literature, it is concluded that in the thylakoid membrane (i) DCBQ binds to Q B, as is generally accepted, and (ii) pBQ binds to the plastoquinol molecules in the PQ pool and most likely also to Q B, thereby in accord with Satoh et al.’s model [K. Satoh, M. Ohhashi, Y. Kashino, H. Koike, Plant Cell Physiol. 36 (1995) 597–605]. An attractive alternative hypothesis is the direct interaction of pBQ with the non-haem Fe 2+ between Q A and Q B.