Pressure to reduce costs has caused a trend toward extended coolant life in heavy duty applications today. Extended life requires the user to maintain the cooling system within tighter limits over a longer period of time. Ideally, the coolant would be replaced based on need, rather than time or mileage. The introduction of corrosive ions can lead to shortened coolant life and the need to replace the coolant sooner than normal in order to protect the engine. In order to know when to replace the coolant, an understanding of the effects of these corrosive ions in the coolant is necessary. This paper focuses on the effects of chloride and sulfate ions on coolant performance in laboratory tests.Laboratory tests were run on a variety of commercially available heavy duty coolants including organic, inorganic, and treated water systems.The tests consisted of various laboratory procedures used throughout the industry to test the ability of a coolant to protect against corrosion. These include (1) ASTM 1384 glassware, (2) vibratory cavitation, (3) ASTM 4340 hot surface, and (4) potentiodynamic electrochemical tests. The testing involved developing a baseline value for each coolant, then altering the coolant test solution by addition of chloride and/or sulfate ions.Various common engine cooling system metals were tested, and a comparison is given to show how the various contaminants in each type of coolant affect the metals.