Abstract: The present invention relates to an improved antifreeze composition for use in engine cooling systems. The composition of the invention has sustained corrosion inhibiting properties even at relatively low concentrations and when used with hard water. The composition comprises an antifreeze agent, an organic acid, a poly(organic acid), dimercapto thiadiazole, a hard water stabilizer, a phosphate salt, a triazole or thiazole and alkali metal hydroxide.