The characterisation of heat-treated sea nodule leached residues by various physicochemical methods (XRD, FT-IR and surface area measurement) and their catalytic activity towards decolourisation of methylene blue (MB) under varying experimental conditions have been studied. Oxidative decolourisation of aqueous MB in batches shows an increase in decolourisation with the increase of catalyst dose, while a decrease in activity is observed with the increase in initial pH of the solution and MB concentration. More than 85% of 10 mg/L MB is decolourised within 30 min with a catalyst dose of 0.6 g/L at pH similar to 3. The effect of different electrolytes on catalytic activity has also been studied.