The standard tests for exhaust system materials are reviewed and new tests involving cyclic oxidation and salt exposure plus oxidation are suggested. The importance of edge oxidation in the interpretation of oxidation weight gain measurement for coated steels is demonstrated. For aluminum-coated steel Type 1, 80% of the weight gain was due to edge oxidation for the test conditions used. Depth profiling an oxidized aluminum-coated steel Type 1 shows why this product is an excellent material for elevated temperature applications. The protective oxide on the aluminum-coated product remains almost pure aluminum oxide even after the coating has alloyed with the base steel. Improved aluminum-coated steels are shown to be possible by changing the chemical composition of the steel substrate.