The photochemistry of the title compounds has been investigated in ethanol and tetrahydrofuran solution under aerobic and anaerobic conditions. Direct irradiation of trans-3,3',4,4'-tetramethoxystilbene (tra~zs-1) in deoxygenated ethanol leads to the rapid establishment of a photostationary state with the cis isomer, and the slower formation of the ethyl ether corresponding to addition of ethanol across the olefinic C=C bond and cyclobutane dimers. The same products are formed upon photolysis in the presence of oxygen under the same conditions but, in addition, two isomeric tetramethoxyphenanthrenes and 3,4-dimethoxybenzaldehyde are formed. Photolysis of trans-1 in oxygenated tetrahydrofuran leads to the same products in different relative yields. Quantum yields for cis,rra~ls photoisomerization, phenanthrene formation, and addition of ethanol have been determined by ferrioxalate actinometry. Direct irradiation of trans-4-hydroxy-3,3',4'-trimethoxystilbene (tratls-2) in ethanol solution also results in rapid cis-tra~ls isomerization and the formation of (three) isomeric phenanthrene derivatives in photolyses carried out in the presence of oxygen, although the material balance is low. The various products of photolysis of trans-2 have been independently synthesized by desilylation of the products isolated from photolysis of trclns-4-tertbutyldimethylsiloxy-3,3',4'-trimethoxystbene (trans-3) under similar conditions. Fluorescence-quenching experiments have been carried out to determine the relative rates of quenching of the excited singlet states of trans-1 and tmtzs-2 by alcohols and oxygen. The formation of aldehydes is proposed to arise via reaction of superoxide ion with stilbene radical cations, which are formed by electron-transfer quenching of the stilbene excited singlet state by oxygen.Key words: stilbenes, lignin, photochemistry, photooxidation.RCsumC : On a CtudiC la photochimie des composCs mentionnks dans le titre en solutions dans 1'Cthanol et le titrahydrofurane, dans des conditions tant akrobiques qu'anakrobiques. L'irradiation direct du tmtzs-3,3',4,4'-tCtramkthoxystilbkne (tratzs-1) dans de 1'Cthanol dCsoxygCnC conduit h la crCation rapide d'un Ctat photostationnaire avec l'isomere cis et h une formation plus lente de l'tther Cthylique correspondant ii l'addition de 1'Cthanol sur la double liaison C=C olCfinique ainsi que des dimkres cyclobutanes. Les m&mes produits se forment lors de la photolyse dans les m&mes conditions, en prCsence d'oxygkne; toutefois, il se forme de plus deux tCtramtthoxyphCnanthrbnes isomkres ainsi que du 3,4-dimCthoxybenzaldthyde. La photolyse du tratzs-1 dans du titrahydrofurane oxygCnC conduit h la formation des m&me produits avec des rendements relatifs diffkrents. On a dCterminC les rendements quantiques de la photoisomCrisation cis, trans, de la formation du phknanthrkne et de l'addition de l'tthanol par actinomCtrie feriioxalate. L'irradiation directe du tmrzs-4-hydroxy-3,3',4'-trimCthoxystilbbne (trans-2) en solution dans 1'Cthanol effectuke en prtsence ...