The title probes are precursors to kinetically calibrated, aryl-substituted cyclopropylcarbinyl radicals that rearrange with picosecond lifetimes. Applications in studies of cytochrome P450-catalyzed hydroxylation reactions are reviewed. Initially confusing results regarding lifetimes of radicals in the hydroxylation reactions were resolved when second-generation probes that distinguish between radicals and cations were employed. The results indicate that two electrophilic oxidizing species are involved in P450-catalyzed hydroxylations, an iron-oxo species that inserts oxygen and a hydroperoxo-iron species that inserts OH(+). The cationic rearrangement products are ascribed to reactions of the protonated alcohol products formed from the latter.