The photodecomposition of 2,5-dimethylfuran vapor sensitized by mercury (3P1) atoms was a complex process in which more than 11 products were formed in characterizable amounts. This number did not include dimeric and condensed materials. The products, in the order of decreasing abundance, were 4-methylcyclopent-2-enone, cis- and trans-1,3-pentadienes, CO, 1-methyl-3-acetylcyclopropene, hexa-3,4-dien-2-one, 2-ethyl-5-methylfuran, isoprene, 1,3-dimethylcyclopropene, 2-pentyne, and propylene. The formation of 2-ethyl-5-methyl-furan was the only process that was totally eliminated on the addition of oxygen. It is hence believed to be the only reaction of free-radical origin. The three minor C5H8 products were shown to be formed by the secondary photolysis of the 1,3-pentadienes. The intermediate that leads to 4-methylcyclopent-2-enone and (CO + 1,3-pentadiene) is best rationalized as a carbene while the intermediate that gives the two other isomeric ketones may be a 1,3-di-radical. Quantum yields and data on pressure quenching have also been obtained. © 1970, American Chemical Society. All rights reserved.