The effects of acetic acid treatment on the physicochemical properties and catalytic activity of manganese nodules have been studied. The surface area, surface oxygen, surface acidity, surface hydroxyl groups, electrical conductivity, etc., increase gradually with acid treatment up to 0.5 M and thereafter show a decreasing trend. The rate constants for H2O2 decomposition and catalytic activity for CO oxidation also follow a trend similar to that of the surface properties.