Hydroxyl radicals were generated radiolytically in N 2 O -and N 2 O / O 2 (4: 1)-saturated aqueous solutions of hyaluronic acid. The hydroxyl radicals react rapidly with hyaluronic acid mainly by abstracting carbon-bound H atom s. As a consequence of subsequent free-radical reactions, chain breakage occurs the kinetics of which has been followed using the pulse radio lysis technique. In the absence of oxygen, strand breakage was followed by the change in conductivity in duced by the release of cationic counterions condensed at the surface of hyaluronic acid which is a polyanion consisting of subunits of glucuronic acid alternating with N-acetyl-glucosamine. It appears that strand breakage is not due to one single first-order process, however, the con tributions of the different com ponents cannot be adequately resolved. At pH7 the overall half-life is 1.4 ms, in both acid and basic solutions the rate of free-radical induced strand breakage is accelerated (at pH 4.8, t 1/2 = 0.6 ms; at pH 10, t 1/2 = 0.18 ms). In the absence of oxygen there is no effect of dose rate on the kinetics of strand breakage. In the presence of oxygen in addition to conductom etric detection, strand breakage was also followed by changes in low-angle laser light-scattering. These two techniques are complementary in that in this system the conductometry requires high doses per pulse while the light-scat tering technique is best operated in the low -dose range. In the presence of oxygen a pro nounced dose-rate effect is observed, e.g. at pH 9.7 after a dose of 9.4 Gy the overall half-time is approx. 0.5 s, while after a dose of 6.6 Gy the half-time is approx. 0.23 s. Both the yield and the rate of strand breakage increase with increasing pH, e.g. at pH 7 G(strand breaks) = 0.7 × 10 -7 mol J -1 and at pH 10.4, 4.8 × 10 -7 mol J -7 . The radiolytic yields of CO 2 , H 2 O 2 , organic hydroperoxides, O 2 ·- and oxygen consum ption have been determined in y-irradiated N 2 O/ 0 2(4: 1)-saturated solutions of both hyaluronic acid and β-cyclodextrin.