11-Arylbenzo[a]fluoren-11-ols I (R = 2-MeC6H4, 3-MeC6H4, 4-MeC6H4, 2-methyl-1-naphthyl, R1 = OH) were prepared by treating benzo[a]fluorenone with arylmagnesium bromides or aryllithiums.Reduction of I (R1 = OH) with HI-HOAc gave I (R1 = H).Both rotational isomers, ap-form of I (R = 2-methyl-1-naphthyl, R1 = OH, II) and 11R-, -sc (or 11S, + sc)-form of I (R = 2-methyl-1-naphthyl, R1 = H) were stable crystals at room temperatureThe rotational barriers (ΔG# = ∼19 kcal/mol) around the Csp3-Csp2 single bond in I (R = 2-MeC6H4, R1 = OH, III) were determined by NMR.Furthermore, the rates of isomerization for II ap → sp and III 11R, -sc (or 11S, +sc) → 11R, + ac (or 11S, -ac) were measured at various temperaturesThe free energies of activation for each process were ΔG#50s = 24.6 kcal/mol and ΔG#170° = 32.8 kcal/mol, resp.