Tricarbonyl-η5-2,4-cyclohexadien-1-yl-manganese (1) was reacted photochemically at 253 K with simple conjugated dienes. Four different types of products were obtained, depending upon the dienes. With 1,3-butadiene (A) dicarbonyl-η4:3-1-(3-buten-1,2-diyl)-2,4-cyclohexadiene-manganese (2A) is isolated. 2-Methyl-1,3-butadiene (B) yields the methyl-substituted diastereomeric dicarbonyls 2B, 2B′, the [4+5]-cycloadduct tricarbonyl-η3:2-3-methyl-bicyclo-[4.3.1]-3,8-decadien-7-yl-manganese (3B) and tetracarbonyl-η3-4-methylene-bicyclo[4.3.1]-8-decen-3-yl-manganese (4B) with an exocyclically coordinated tetracarbonylmanganese fragment. With 2,3-dimethyl-1,3-butadiene (C) only the [4+5]-cycloadduct 3C and the tetracarbonyl 4C are obtained. No CC-bond formation is observed with E,E-2,4-hexadiene (D) and 1,3-cyclohexadiene (E). Carbonyl-η5-2,4-cyclohexadien-1 -yl-η4-E,E-2,4-hexadiene-manganese (5D), and carbonyl-η4-1,3-cyclohexadiene-η5-2,4-cyclohexadien-1-yl-manganese (5E) are the only products. The complexes were separated and purified by HPL chromatography. Their constitutions were determined by IR and NMR spectroscopy.