Atmospheric aerosol has been shown to contain an organic component that includes a significant fraction of small dicarboxylic acids, particularly in the urban environment. As an initial step toward understanding the phase in which particles may exist, a detailed study into the phase transitions of malonic and oxalic acid aerosols has been carried out. Both the aerosol phase transitions (deliquescence and efflorescence) and bulk solution properties (equilibrium water vapor pressure and the solubility and freezing curves of the aqueous solutions) are reported. An aerosol flow tube-FTIR and a static mode chamber-FTIR have been used to identify particulate phase transitions. In the latter the particles can be observed under ice-supersaturated conditions, allowing investigation of behavior at subeutectic temperatures. We report that both malonic and oxalic acid aerosols sustain a substantial level of solute supersaturation before efflorescence occurs, whereas deliquescence occurs at the thermodynamically predicted relative humidity. At room temperature, malonic acid efflorescence is observed at RH = 6% +/- 3% and oxalic acid efflorescence occurs at RH less than or equal to 5%. Malonic acid particles deliquesce between 69% and 91% RH over the temperature range 293-252 K, and for oxalic acid conditions close to 100% RH are required. We report the first observation of the phase transition of oxalic acid between the anhydrous and dihydrate form and discuss our results in the context of recently published data.