Nonlocal gradient-corrected and hybrid density functional theory (DFT) have been used to calculate T1 potential energy surfaces (PES), spin densities, and geometries of ethylene and aromatic olefins of various sizes: ethylene (1), styrene (2), stilbene (3), 1,1 -diphenylethylene (4), 1,4-bis-(1-propenyl)benzene (5), 1,3-divinylbenzene (6), and 2-(1-propenyl)anthracene (7). Calculated properties were used to determine differences in electronic structure of olefins that follow adiabatic vs diabatic Z/E-isomerization mechanisms. In the planar TI structure. the C=C bond in 1 is elongated to a single bond, but in 7 it remains a double bond, archetypal of excitations in the olefinic bond and in the substituent, respectively. Changes in geometries and spin-density distributions of 2-7 reveal that substituent aromaticities vary along the Tl PES. For systems that isomerize diabatically (e.g., 2), substituent aromaticity is regained in the 90 degrees twisted structure of the C=C bond (3p*). This leads to stabilization and a minimum on the PES at 3p*. If the substituent of the planar T1 olefin fully can accommodate the triplet biradical and still remain aromatic as in 7, aromaticity is instead reduced upon twist to 3p*, SO that the T1 PES has a barrier that is suitable for adiabatic isomerizations. The planar structures of olefins with substituents that are partially antiaromatic in Ti (e.g., phenyl) can be stabilized by radical accepting groups in the proper positions (e.g., 5). In summary, our calculations indicate that for an aryl-substituted olefin the structure with the highest substituent aromaticity in TI corresponds to the minimum on the TI PES of Z/E-isomerizations.