Biphenyl-2-ylcarbenes, 2-ArC6H,CR, were generated photolytically and thermally from diazo precursors. Cyclization, leading to fluorenes, competes with capture of the carbenes by methanol but proceeds faster than intramolecular hydrogen shifts (with R = Me) and intermolecular C-H insertion reactions (with R = H in cyclohexane). By comparison of product ratios with kinetic data for related carbenes from the literature, the cyclization rate is estimated as cu 10" s-'. The intramolecular reactivity of biphenyl-2-ylcarbenes is not significantly attenuated by variation of R (R = H, Me, Ph). Very minor effects of triplet sensitization and methanol quenching indicate that fluorenes arise from spin-equilibrated biphenyl-2-ylcarbenes, presumably from the singlet state. When A r = mesityl, the carbene predominantly inserts into C-H bonds of the 2'-methyl groups, giving rise to a dihydrophenanthrene. Formation of a fluorene derivative, by formal insertion into C-C bonds, occurs as a minor process. This unprecedented reaction points to intervention of an o-xylylene in which the methyl group migrates. Laser flash photolysis (LFP) of 2-PhC6H,CN,Ph generates a transient absorption which is due to the To -+ T, transition of 9-phenylfluorene rather than to the presumed o-xylylene. On LFP of 2-ArC,H,CN,Ph in trifluoroethanol-acetonitrile, protonation of the carbenes gives rise to carbocations, 2-ArC,H,CH 'Ph. The transient absorption spectra of these cations are strongly influenced by twisting about the Ar-Ar bond (Ar = P h i o-tolyl< mesityl) whereas the rates of nucleophilic capture vary only slightly. Biphenyl-2-ylcarbenium ions (Ar = R = Ph) cyclize more slowly than the analogous carbenes, by a factor of 310,.