Particles containing potassium salts are common in biomass burning plumes and include compounds such as KCl, KNO 3 , and K 2 SO 4 . We determined the deliquescence and efflorescence relative humidities of single- and multi-component potassium salts using an environmental transmission electron microscope and an environmental scanning electron microscope. KCl particles deliquesced at a relative humidity (RH) of 85% and effloresced at 56%. K 2 SO 4 particles deliquesced at 96% and effloresced at 60%. KNO 3 particles grew continuously with increasing humidity. If KNO 3 particles were heat-treated at 104°C overnight, they deliquesced at an RH above 90%. Atomized solutions of multi-component potassium compounds (KCl & KNO 3 and KCl & K 2 SO 4 ) formed particles that contained distinct grains of each compound upon drying. Particles formed from the KCl & KNO 3 mixtures grew continuously with increasing humidity until deliquescence occurred at an RH of 82% and efflorescence occurred at 58%. Particles from the KCl & K 2 SO 4 mixture began to take up water at an RH of 84%. They deliquesced at >90% and effloresced at 52%. This work provides new data for mixed aqueous electrolytes that are important for atmospheric science. Efflorescence values for K 2 SO 4 , KNO 3 , and internally mixed particles of K 2 SO 4 and KCl are reported here for the first time.