Thecis-andtrans-isomers of fourpara-substituted (three disubstituted and one tetrasubstituted) tetraphenylethylenes have been synthesized, separated, and characterized by either dipole moment or X-ray analysis. The rates of thermal isomerization of the disubstituted derivatives in benzene solution correlate with a substituent parameter reflecting substituent effects on benzylic radical stabilities (σ•), which reflects the biradical nature of the transition state for thermal isomerization in this system. A significant rate enhancement is observed for the tetrasubstituted derivative, in which merostabilization of the transition state is possible.The Arrhenius parameters for thermal isomerization in benzene were determined. From the variation observed for the disubstituted derivatives, an estimate of 35.5 kcal mol−1for the rotational barrier in tetraphenylethylene is obtained. The activation energy for thermal isomerization of the tetrasubstituted derivative is 2 kcal mol−1lower than those of the other three; this provides an assessment of the importance of merostabilization in this system.The thermal isomerization of some related olefins (e.g. bianthrylidenes and indigo derivatives) is discussed in light of these results.