The present invention relates to a technique for improving the tarnish and oxidation resistance of copper and copper base alloy materials. The technique of the present invention involves immersing the copper or copper base alloy material in a dilute aqueous chromic acid-phosphoric acid solution. After emerging from the chromic acid-phosphoric acid solution, the copper or copper base alloy material is preferably rinsed with a dilute aqueous caustic solution and dried. Copper and copper base alloy materials treated in accordance with the present invention have particular utility in printed circuit applications.