+ H 2 O over the full composition range, along with those of the single salt systems.Boiling temperatures were also measured for the four component NaCl + NaNO 3 + KNO 3 + H 2 O and five component NaCl + NaNO 3 + KNO 3 + Ca(NO 3 ) 2 + H 2 O mixtures, where the solute mole fraction of Ca(NO 3 ) 2 , x{Ca(NO 3 ) 2 }, was varied between 0 and 0.25. The maximum boiling temperature found for the NaCl + KNO 3 + H 2 O system is ≈134.9 °C; for the NaNO 3 + KNO 3 + H 2 O system is ≈165.1 °C at x(NaNO 3 ) ≈ 0.46 and x(KNO 3 ) ≈ 0.54; and for the NaCl + Ca(NO 3 ) 2 + H 2 O system is 164.7 ± 0.6 °C at x{NaCl} ≈ 0.25 and x{Ca(NO 3 ) 2 } ≈ 0.75. The NaCl + NaNO 3 + KNO 3 + Ca(NO 3 ) 2 + H 2 O system forms molten salts below their maximum boiling temperatures, and the temperatures corresponding to the cessation of boiling (dry out temperatures) of these liquid mixtures were determined. These dry out temperatures range from ≈300 °C when x{Ca(NO 3 ) 2 } = 0 to ≥ 400 °C when x{Ca(NO 3 ) 2 } = 0.20 and 0.25. Mutual deliquescence/efflorescence relative humidity (MDRH/MERH) measurements were also made for the NaNO 3 + KNO 3 and NaCl + NaNO 3 + KNO 3 salt mixture from 120 to 180 °C at ambient pressure.The NaNO 3 + KNO 3 salt mixture has a MDRH of 26.4% at 120 °C and 20.0% at 150 °C.This salt mixture also absorbs water at 180 °C, which is higher than expected from the boiling temperature experiments. The NaCl + NaNO 3 + KNO 3 salt mixture was found to have a MDRH of 25.9% at 120 °C and 10.5% at 180 °C. The investigated mixture compositions correspond to some of the major mineral assemblages that are predicted to control brine composition due to the deliquescence of salts formed in dust deposited on waste canisters in the proposed nuclear repository at Yucca Mountain, Nevada. 2KEY WORDS: Boiling temperature; deliquescence relative humidity; efflorescence relative humidity; saturated solutions; sodium chloride; sodium nitrate; potassium nitrate; calcium nitrate; aqueous solutions.