One stereoisomer of 1-(E-styryl)-2-phenylcyclopropane undergoes reversible thermal stereomutations to equilibrate with three other stereoisomers and gives more slowly the four isomers of 3,4-diphenylcyclopentene. Appropriate kinetic data and analyses have provided rate constants for the four stereochemically distinct paths leading from one enantiomer of trans-1-(E-styryl) -2-phenylcyclopropane to 3,4-diphenylcyclopentenes : at 160 OC the relative significances of the four distinct paths followed in these vinylcyclopropane to cyclopentene rearrangements are (67 +/- 3)% si, (12 +/- 3)% ar, (17 +/- 1)% sr, and (4 +/- 1)% ai. The orbital symmetry "allowed" si and ar paths are phenylcyclopropane isomerizes to 3,4-diphenylcyclopropanes at comparable rates, with the "forbidden" paths favored over the "allowed" routes, 91:9. The rearrangements may be considered to be processes mediated by kinetically competitive diradical transition structures rather than controlled by orbital symmetry considerations.