We analyze conformational stereoisomerism in the sterically hindered 4a,4b-dihydrophenanthrenes of C-2 symmetry. In these labile photointermediates, steric hindrance brought about by disubstitution at the 4 and 5 positions or by benzoannelation at the [c] and [g] bonds allows observation of two interconvertible intermediates denoted as L and S. In such molecules, theoretical analysis suggests two low-energy structures (C and T). These differ in the chirality of the polyene perimeter helix for one given configuration of the 4a,4b-ethane unit. C and T can be distinguished by the position of atom pair H-4a, X-4, or Of the equivalent atom pair H-4b, X-5 With respect to the the mean molecular plane. In C, atoms H-4a and X-4 are on one side of the mean molecular plane, while in T atoms H-4a and X-4 are on opposite sides of this plane. The same holds for the atom pair H-4b, X-5. The C conformation is assigned to the primary photocyclization product (L modification) in which the intersubstituent distance X-4-X-5 is short and where the angular hydrogens are quasi-antiperiplanar. This is the regular conformation of the unhindered 4a,4b-dihydrophenanthrenes. The T conformation is assigned to the secondary conformers (S modification) formed spontaneously from the primary L product. Here the distances H-4a-X-5 and H-4b-X-4 are short, and in the case of severe steric hindrance (X = t-Bu) the angular hydrogens are anticlinal. The observed stability order (usually S > L at around room temperature, except in the 4,5-dimethyl, 4,5-di-tert-butyl, and 4,5-dichloro molecules) is the outcome of steric hindrance opposing skeletal deformations. X-4-X-5 steric interactions are dominant in the C forms while the H-4a-X-5 and H-4b-X-4 interactions (and corresponding interactions in the [c] and [g] dibenzoannelated systems) play a foremost role in the T forms. Computed strain energies for C and T reproduce the observed stability trends of L and S modifications. Differences in visible absorption band energies are traced to different extents of departure from planarity of the polyene perimeter. The L intermediates assigned to the more planar C structure are thus predicted to absorb at longer wavelengths. The low intensity of the visible band in the L modifications is attributed to symmetry-imposed cancelling-out of one-ethylene-bond transition moments. The present analysis explains also the simultaneous existence and the observed range of values of potential barriers separating the two modifications. The present analysis changes the previous structure assignment for the S and L modifications of the dibenzo system, leading to full agreement with the observed trend of the electronic spectra of the S and L modifications. Similar considerations applied to the 4a,4b-dimethyl system suggest that its labile low-temperature intermediate (lambda(max) = 360 nm) could have a T-like conformation.