pi-Bond shift isomerism in bicyclo[4.2.2]decapentaene provides two unique structures, [4]paracyclophane-1,3-diene 1 and 1,6-ethenocycloocta-1,3,5,7-tetraene 2. The photochemical transformation of [4.2.2]propella-2,4,7,9-tetraene (7a) and its 7-(methoxycarbonyl) derivative (7b) into the corresponding bicyclo[4.2.2]decapentaenes has been examined. Irradiation of 7a in MeOH affords an adduct of MeOH whose formation suggests the generation of bicyclo[4.2.2]decapentaene, but does not permit differentiation between the structural alternatives 1a and 2a. Photolysis of 7a in the presence of cyclopentadiene provides adducts whose structures demonstrate that the intermediate is trapped in the form of 1. The structures of cyclopentadiene adducts derived from 7b also indicate the generation of the intermediate in the form of 1. In no case was an adduct derived from the interception of the intermediates in the form of 2 detected. Electronic absorption spectra observed upon irradiation of 7a and 7b at 77 K are remarkably similar in shape to those of corresponding [4]paracyclophanes and thus substantiate again that the phototransformation of 7a and 7b leads to species whose structures are best represented by the form of 1. The results of the geometrical optimization of 1a and 2a undertaken at the SCF level employing 3-21G and 6-31G* basis sets and also using semiempirical MNDO, AM1, and PM3 Hamiltonians are presented. The semiempirical methods as well as the ab initio calculations at 3-21G and 6-31G* levels predict that 1a is 5.4-12.2, kcal/mol less stable than 2a. With the application of the MP2 correlation energy treatment, however, the energy of la relative to 2a is remarkably lowered and 1a is predicted to lie 5.5-4.7 kcal/mol below 2a. The preparation of 7a and 7b from tetrahydrocinnamic acid in overall yields of 4.9% and 5.9%, respectively, is described.