Isolated chloroplast lamellae from spinach or sugar-beet leaves produce ethylene from methylmercaptopropanal (MMP) or 2-keto-4-methylmercaptobutyrate (KMB) in the light. The pH-optimum for ethylene production from M M P is 6.3; ethylene production from KMB has an apparent pH-optimum at about pH 5. Ethylene formation in red light (90 kerg ·cm -2 -sec -1 ) from the above substrates is stimulated by ferredoxin and inhibited by either DCMU (10 -5 м), ferredoxin together with NADP, catalase, superoxide dismutase or under anaerobic conditions. From the inhibition by either DCMU, NADP or anaerobic conditions it is concluded, that an intact electron transport system from water as electron donor to oxygen as electron acceptor is necessary for ethylene form ation. H 2 O 2 alone does not stimulate ethylene formation from M M P or KMB. Inhibition by both catalase and superoxide dismutase support the view, that ethylene formation from M M P or KMB is driven by the OH-radical, which is formed from H 2 O 2 and the superoxide free radical ion. The presented data suggest that in addition to ferredoxin another membrane-bound factor is involved in photosynthetic oxygen reduction and ethylene formation. This factor (ORF = oxygen reducing factor) stimulates photosynthetic oxygen reduction in the presence of ferredoxin yielding H 2 O 2 , in addition to O 2 .- , which is the product of the autooxidation of reduced ferredoxin. During photosynthetic ethylene formation from MMP or KMB, the production of the OH-radical from H 2 O 2 and O2 .- according to H 2 O 2 + O 2 ·- → OH · +OH - +O 2 (Haber and Weiss, Proc. Roy. Soc. Ser. A 147, 332 , Beauchamp and Fridovich, J. Biol. Chem. 245, 4641 ) seems to be the rate-limiting step.