An aqueous solution of glucuronic or galacturonic acid was treated in a continuous tubular reactor under subcritical conditions. The antioxidative capacity of the degradation products was estimated by three assays. The DPPH radical-scavenging activity of the degradation products was measured. For both substrates, the DPPH radical-scavenging activity and substrate conversion increased with increasing residence time in the reactor. Glucuronic and galacturonic acid were treated in subcritical water and subsequently lyophilized. The lyophilized degradation products were investigated for their antioxidative activity against the oxidation of lioneic acid under accelerated conditions at 110 degrees C. The respective degradation products of glucuronic and galacturonic acid did not exhibit any antioxidative activity in linoleic acid, as reflected by the unaltered oxidation induction period. However, the lyophilized degradation products of the substrates effectively inhibited the oxidation of linoleic acid in aqueous dispersion initiated by the water-soluble initiator AAPH at 45 degrees C, by prolonging the oxidationinduction period and decelerating the oxygen consumption rate in the propagation period.