We have studied the inhibition of photosynthetic electron transport by UV-A (320–400 nm) radiation in isolated spinach thylakoids. Measurements of Photosystem II (PSII) and Photosystem I activity by Clark-type oxygen electrode demonstrated that electron flow is impaired primarily in PSII. The site and mechanism of UV-A induced damage within PSII was assessed by flash-induced oxygen and thermoluminescence (TL) measurements. The flash pattern of oxygen evolution showed an increased amount of the S₀ state in the dark, which indicate a direct effect of UV-A in the water-oxidizing complex. TL measurements revealed the UV-A induced loss of PSII centers in which charge recombination between the S₂ state of the water oxidizing complex and the semireduced Q_A⁻ and Q_B⁻ quinone electron acceptors occur. Flash-induced oscillation of the B TL band, originating from the S₂Q_B⁻ recombination, showed a decreased amplitude after the second flash relative to that after the first one, which is consistent with a decrease in the amount of Q_B⁻ relative to Q_B in dark adapted samples. The efficiency of UV-A light in inhibiting PSII electron transport exceeds that of visible light 45-fold on the basis of equal energy and 60-fold on the basis of equal photon number, respectively. In conclusion, our data show that UV-A radiation is highly damaging for PSII, whose electron transport is affected both at the water oxidizing complex, and the binding site of the Q_B quinone electron acceptor in a similar way to that caused by UV-B radiation.