Finely divided composite materials are provided comprising a first metal oxide (e.g., MgO) at least partially coated with an extremely thin layer of a second metal oxide such as Fe.sub.2 O.sub.3. The composites have a high surface area and are very effective for the destructive adsorption of undesirable fluids in gaseous or liquid form, such as chlorocarbons and chlorofluorocarbons. In use, a fluid stream including undesirable fluids are contacted with the composites of the invention, such as through the use of a filter containing the composite as a part of the filter media thereof.