Azelaic acid is an aliphatic dicarboxylic acid (HOOC-(CH2)7-COOH) which has recently been shown to have some practical therapeutic applications in skin diseases of different etiologies. It possesses diverse biological activities and its mechanisms of action are still under investigation. Azelaic acid, as disodium salt (C(9)2Na), at concentrations from 0.05 mM to 1.0 mM is capable of inhibiting significantly the hydroxylation of 1-tyrosine to 1-DOPA due to hydroxylradicals (HO.) produced by Fenton reaction. Similarly C(9)2Na significantly inhibits the heterogeneous photocatalytic oxidation of toluene to cresols, and the peroxidation of arachidonic acid (C20:4,n6), due to HO. formed by dissolved oxygen in the presence of UV-irradiated semiconductor TiO2 (photo-Fenton type reaction). C(9)2Na decomposition and its by-products formation are quantifiable only at high HO. concentrations. On the contrary, C(9)2Na is not a scavenger of O2-. generated by xanthine-xanthine oxidase system. Under the same experimental conditions, mannitol behaves like C(9)2Na. These data indicate that HO. scavenging capacity of C(9)2Na in vitro, and represent a useful tool for further investigations on the mechanisms of action of azelaic acid in biological systems.