X-Ray crystal analysis has been used to identify one isomer (III) of the two products C9H10O4{methyl 2,5-epidioxybicyclo[4.1.0]hept-3-ene-7-carboxylate (II) and methyl 2α,3α:4α,5α-diepoxy-cis(1αH,2αH)-bicyclo[4.1.0]heptane-7α-carboxylate (III)} produced by the photosensitized oxygenation of methyl cyclohepta-2,4,6-trienecarboxylate (I). Crystals of (III) are orthorhombic, space group P212121, with a= 19.548(1), b= 6.0101 (5), and c= 7.3944(4)Å and Z= 4. By hydrogenation of (II), methyl 2,5-dihydroxybicyclo[4.1.0]heptane-7carboxylate (V) is formed which also crystallizes in space group P212121 with Z= 4, and cell dimensions a= 18.192(1), b= 7.7284(5), and c= 6.7139(3)Å. The structures were solved by the tangent formula phasing method and refined by full-matrix least-squares to R 0.055 (962 observed reflections) and 0.044 (1 028 observed reflections) respectively.