The reactions of OH., O.-, and SO4.- with 1,3,7-trimethylxanthine (caffeine) were studied by pulse radiolysis with optical and conductance detection techniques. The absorption spectra of transients formed in OH. reaction in neutral solutions exhibited peaks at 310 and 335 nm, as well as a broad absorption maximum at 500 nm, which decayed by first-order kinetics. The rate (k = (4.0 +/- 0.5) x 10(4) s(-1)) of this decay is independent of pH in the range 4-9 and is in agreement with that determined from the conductance detection (k = 4 x 10(4) s(-1)). The spectrum in acidic solutions has only a broad peak around 330 nm with no absorption in the higher wavelength region. The intermediates formed in reaction of O.- absorb around 310 and at 500 nm, and the first-order decay at the latter wavelength was not seen. The OH radical adds to C-4 (X-4OH(.)) and C-8 (X-8OH(.)) positions of caffeine in the ratio 1:2 as determined from the redox titration and conductivity measurements. H abstraction from the methyl group is an additional reaction channel in O.- reaction. Dehydroxylation of the X-4OH(.) adduct occurs, whereas the X-8OH(.) adduct does not undergo ring opening. The rate constant for addition of O-2 to X-4OH(.) is estimated to be similar to 1 x 10(9) M-1 s(-1), whereas it is unreactive toward X-8OH(.). The spectrum obtained for OH. reaction in oxygenated solutions is similar to that observed in SO4.- reaction in basic solutions. The radical cation of caffeine formed from its reaction with SO4.- (lambda(max) = 320-340 nm) is hydrolyzed in basic solutions to yield the X-8OH(.) adduct. The molar absorptivities of the X-8OH(.) and the X-4OH(.) adducts at 300 and 335 nm are 6500 and 5300 M-1 cm(-1), respectively. The yield of 1,3,7-trimethyluric acid in OH. reaction in the absence of O-2 (28%) is reduced by more than 50% in its presence. The differences in the mechanism of OH. reaction with caffeine and its isomer 1,3,9-trimethylxanthine (isocaffeine) are discussed.