Scanning electron microscopy and microchemical analyses were conducted to determine the electrode erosion mechanisms of two different spark plugs, platinum-tip and nickel-alloy plugs, tested under various environments. The electrodes were eroded differently depending on the electrode materials and the operating environments. In bench test performed in a dry air pressure chamber, ejection of small molten particles from center electrodes and vaporization loss of side electrodes were the predominant erosion processes. In vehicle tests, the erosion mechanisms varied with the additives in the fuels used. Oxidation of base metal and electrode corrosion by fuel residues were the common erosion processes observed.