kappa-Carrageenan was degraded by an oxidative method involving hydrogen peroxide (H2O2). The molecular weight was measured by gel permeation chromatography. The effects of the concentration of H2O2 and initial kappa-carrageenan, pH value, and degradation time on the molecular weights of the degraded products were studied. The structural change of the degraded kappa-carrageenans was characterized by Fourier transform infrared spectroscopy and determination of the sulfate content. The antioxidant activity of the degraded kappa-carrageenans was evaluated as scavengers of superoxide anions and hydroxyl radicals by application of flow injection chemiluminescence technology. The values of the 50% inhibition concentration (IC50) against the superoxide anion of degraded kappa-carrageenans labeled A, B, C, and D (with weight-average molecular weights of 3250, 5820, 15,080, and 209,000, respectively) were 2.65, 3.22, 6.66, and 8.13 mg/mL, respectively. As for hydroxyl radical scavenging, the IC50 values of kappa-carrageenans A, B, C, and D were 0.014, 0.049, 0.062, and 0.110 mg/mL, respectively. The results indicated that the kappa-carrageenans with lower molecular weights had better antioxidant activity. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 194-199, 2010