We have investigated the thermodynamics and kinetics of ammonium nitrate/water and mixed ammonium nitrate/succinic acid/water microparticles. The water activity of ammonium nitrate microparticles is determined as a function of composition down to 12% relative humidity by accounting for the rapid evaporation of ammonia and nitric acid. Both the observed deliquescence and water activities for ammonium nitrate/water microparticles are found to be in good agreement with predicted values. Supermicron ammonium nitrate particles generated from high-purity methanol are found not to effloresce, but form an anhydrous liquid state observed only in particle form. We also present data on the hydroscopicity and phase transitions of internally mixed ammonium nitrate/succinic acid particles. A pronounced reduction in the particle growth factor at deliquescence is reported, indicating that the succinic acid does not take up a significant amount of water. Additionally, the deliquescence relative humidity is found to decrease slightly as the mass percent of succinic acid is increased from 12.5 to 50%. Solid succinic acid in ammonium nitrate drops acts to catalyze efflorescence of ammonium nitrate at high relative humidity. However, when the relative humidity is increased sufficiently to completely dissolve the succinic acid, no efflorescence is observed. The resulting data are analyzed using classical nucleation theory to derive the free energy barrier to nucleation, the critical supersaturation, the size of the critical nucleus, and the contact parameter between ammonium nitrate crystal and the solid succinic acid.