Hydroxyl radical reacts readily with beta-carotene following submicro-second laser photolysis of N-hydroxypyridine-2(1H)-thione (N-HPT) as a "photo-Fenton" reagent generating hydroxyl and thiyl radicals in acetonitrile:tetrahydrofuran (4:1, v/v) solution. On the basis of spectral evidence, and supported by kinetic considerations and thermodynamic calculations, a short-lived transient species, detected by time-resolved absorption spectroscopy with an absorption maximum at similar to 750 nm and a lifetime of similar to 150 ns at 25 degrees C under anaerobic conditions, is suggested to be the long-sought neutral beta-carotene radical formed by hydrogen-atom abstraction. The transient spectrum is different from the spectra of the beta-carotene radical cation (similar to 1000 nm absorption maximum with a millisecond lifetime), the beta-carotene radical adducts (similar to 520 nm, several microsecond lifetime), the beta-carotene radical cation ion pair (similar to 750 nm, several hundred microsecond lifetime), and the beta-carotene radical anion (similar to 880 nm, a few tens of microsecond lifetime). In parallel, beta-carotene reacts with the thiyl radical to yield a sulfur radical adduct with absorption maximum at similar to 520 nm with a lifetime of 3.0 mu s. For astaxanthin and canthaxanthin, the reaction with the thiyl radical dominates and the neutral radical is hardly formed in agreement with the less reducing properties of these 4,4'-diketo carotenoids without the reactive 4,4'-hydrogens.