Today's automotive engine coolants must give corrosion protection in both the standard cast-iron and brass and the newer aluminum cooling systems. The formulation of coolants for this dual protection was studied by investigating the effects of various corrosion inhibitors, pH, and coolant silicate depletion on cooling system corrosion. Laboratory glassware and simulated service corrosion tests were used for the study. It was found that inhibitors for some metals can be corrosive to others, but good aggregate protection for both types of cooling systems can be obtained by using some combinations of common coolant corrosion inhibitors. Low coolant pH is to be preferred over high pH for cooling systems containing aluminum. Coolant depleted of silicate is dependent on the remaining inhibitors for ongoing aluminum protection, and little or no accrued protection results from the prior contact by silicate. Several coolant formulations are described that give good aggregate protection without resort to phosphate or silicate.