AbstractDipyridamole [2,6‐bis‐diethanolamino‐4,8‐dipiperidinopyrimido‐(5,4‐d) pyrimidine], a well known platelet aggregation inhibitor, shows powerful hydroxyl radical scavenging activity by inhibiting OH‐dependent salicylate and deoxyribose degradation. Steady‐state competition kinetics experiments with deoxyribose were carried out to evaluate the second‐order rateconstant for the reaction between hydroxyl radical and dipyridamole. OH· radicals were generated either by a Fenton‐type reaction or by X‐ray irradiation of water solutions. A second‐order rate constant k(Dipyridamole+OH·) of 1.72±0.11×1010M−1 s−1 and of 1.54±0.15×1010 M−1 s−1 was measured by Fenton chemistry and by radiation chemistry, respectively. Mannitol was used as an internal standard for hydroxyl radicals in steady‐state competition experiments with deoxyribose. A rate constant k(Mannitol+OH·) of 1.58±0.13×109 M−1 s−1 and 1.88±0.14×109 M−1 s−1 was measured in the Fenton model and in the water radiolysis system, respectively. Both these rate constants are in good agreement with the published data obtained by the “deoxyribose assay” and by pulse radiolysis.