Plastoquinol (PQH 2 -9) and plastoquinone (PQ-9) mediate photosynthetic electron transfer. We isolated PQH 2 -9 from thylakoid membranes, purified it with HPLC, subjected the purified PQH 2 -9 to singlet oxygen ( 1 O 2 ) and analyzed the products. The main reaction of 1 O 2 with PQH 2 -9 in methanol was found to result in formation of PQ-9 and H 2 O 2 , and the amount of H 2 O 2 produced was essentially the same as the amount of oxidized PQH 2 -9. Formation of H 2 O 2 in the reaction between 1 O 2 and PQH 2 -9 may be an important source of H 2 O 2 within the lipophilic thylakoid membrane.