9,10-Diphenyl-1,2,3,4-tctrahydro-2,1,4-othanylylidenenaphthalalie (II) isomerizes at 160-250� to give 1-(cis-1,2-diphenylethenyl)-1,2- dihydronaphthalene (IV). The structure of (IV) is supported by its p.m.r., u.v., and mass spectra, as well as by its conversion into 1- (1,2-diphenylethyl)naphthalene (V) and into 1-(cis-1,2- diphenylethenyl)naphthalene (VII) by palladium and palladium- nitrobenzene, respectively. Compounds (V) and (VII) have been synthesized independently from 1-(1-naphthyl)-1,2-diphenylethanol (VI).