The objective of this study was to develop an accelerated aging test (AAT) for conventional and extended life coolants that will predict coolant composition and performance after 100 000 or more miles (160 930 km) of use. The procedure was developed by examining the effects of a series of cooling system metals, their surface area and the amount of each used, test temperature, glycol concentration, and test time on important chemical and physical properties of the test coolant. The chemical and physical properties evaluated included the accumulation of glycol degradation products, the depletion rate of active inhibitors, the pH drop, and the presence of corrosion products in solution. In addition, the test coolant performance was evaluated in ASTM D 1384 and D 4340. The effects of variation in the test procedure on the coolant were compared to actual coolant from extended duration fleet tests. The test procedure selected gave test coolant with composition, physical properties, and performance that compared favorably with the fleet test fluid. The test performance was validated by comparing the properties of a series fluids after this test to corresponding fluids removed from vehicles after extended use. An example of fluid development using this procedure is presented. Further areas of investigation are suggested. It is recommended that the general test procedure be considered for adoption as an ASTM test method for evaluation of the extended performance of fluids in automotive and light duty cooling systems.