The promoting effect of Ag2O on the physicochemical properties and catalytic activity of manganese nodules has been investigated. The surface area and surface oxygen increased with addition of Ag2O, reaching a maximum at 1 mol% of Ag, and then decreased with further addition of Ag2O. The catalytic activity of manganese nodules substantially increased with addition of Ag2O over a wide range (0.25-25 mol% Ag). Addition of only 0.25 mol% of Ag to the nodule decreased the temperature of complete CO oxidation from 350 degrees C to as low as 100 degrees C. This enhanced activity was explained by the concerted action of Ag and Mn, where CO was preferentially adsorbed on Ag2O and oxidized. The reduced Ag was then reoxidized rapidly by oxygen transferred from Mn. The promoting effect of Ag2O on H2O2 decomposition and ketonization of acetic acid, however, was less pronounced,