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Cyclopropylbenzyl radicals formed by pulse radiolysis of cyclopropylbenzyl benzoates undergo ring-opening rearrangements with absolute rate constants (0.5-4) x lo5 s-1 in hexane at room temperature.Ring-opening of cyclopropylmethyl radical (1) to the but-3enyl radical [equation (l), R1 = R2 = HI has been used extensively as a radical clock.1 The rate of rearrangement k depends upon substituents. Ring-opening of radical (2) (R1 = H, R2 = Me)2 is slower than that of primary radical (1).1 The -40 kJ mol-1 resonance stabilization energy of benzylic radicals3 suggests that k would be even lower for cyclopropylbenzyl radicals, but the rate of rearrangement of a benzylic radical has not been established. We report that cyclopropylbenzyl radicals may be observed at room temperature by transient absorption techniques. Direct observation of these radicals under ambient conditions obviates an extrapolation of the rate constants from low temperature measurements. 1 The absolute rate constants k found for benzylic radicals (3)-( 7) correlate with Creary's (T' parameter.4The l-cyclopropyl-l-phenylethyl radical (8) (R1 = Ph, R2 = Me), when formed by hydrogen atom transfer to a-cyclopropylstyrene, has been observed to rearrange according to equation (1) in competition with hydrogen atom abstraction from transition metal hydrides.5 Alkyl radicals are also produced effectively by pulse radiolysis of alkyl esters of carboxylic acids .6 Pulse radiolysis of benzhydryl benzoate in hexane under a nitrogen atmosphere affords absorptions near 330 nm, which are characteristic of the diphenylmethyl radical7 [Figure l(a)]. The radical decays via second-order processes to form coupling products.6 Pulse radiolysis of