It is now well recognized that mixed organic–inorganic particles are abundant in the atmosphere. While there have been numerous studies of efflorescence of mixed organic–inorganic particles close to 293 K, there are only a few at temperatures less than 273 K. Understanding the efflorescence properties of these particles at temperatures less than 273 K could be especially important for predicting ice nucleation in the upper troposphere. We studied the efflorescence properties of mixed citric acid–ammonium sulfate particles as a function of temperature to better understand the efflorescence properties of mixed organic–inorganic particles in the middle and upper troposphere. Our data for 293 K illustrate that the addition of citric acid decreases the ERH of ammonium sulfate, which is consistent with the trends observed with other systems containing highly oxygenated organic compounds. At low temperatures the trend is qualitatively the same, but efflorescence can be inhibited by smaller concentrations of citric acid. For example at temperatures <250 K an organic mass/(organic mass + sulfate mass) of only 0.33 is needed to inhibit efflorescence of ammonium sulfate. In the upper troposphere the organic mass/(organic mass + sulfate mass) can often be larger than this value. As a result, particles in the upper troposphere may be more likely to remain in the liquid state than previously thought and solid ammonium sulfate may be less likely to participate in heterogeneous ice nucleation in the upper troposphere. Additional studies are required on other model organic systems.