An aqueous heat transfer fluid comprising a C.sub.3 carboxylate freezing point depressant, either solely or in combination with C.sub.3-C.sub.5 carboxylate freezing point depressants, in combination with C.sub.6-C.sub.16 organic acid salt corrosion inhibitors, provides an antifreeze/coolant composition wherein the corrosion inhibitors exhibit uniquely enhanced solubility, especially at low temperatures, in synergistic combination with the selected carboxylate freezing point depressants.