The effects of alkali (sodium hydroxide) treatment on the physico-chemical properties and catalytic activity for H2O2 decomposition and CO oxidation of Indian Ocean manganese nodules have been studied. The surface area, surface oxygen, surface hydroxyl groups, etc. increase with alkali treatment up to 0.05 M and there after show a decreasing trend. The high catalytic activity of 0.05 M NaOH treated samples are correlated with the surface properties.