Molar enthalpies of crystallization of (2S,5R,6R)-6-amino-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid in sodium sulfate and ammonium sulfate aqueous solutions were estimated based on the solubilities. A positive effect of temperature on solubility of 6-APA in both Na(2)SO(4) and (NH(4))(2)SO(4) aqueous solutions gave a result or exothermic crystallization process, and the enthalpy released was about (7 to 13) kJ.mol(-1). Concentrations of the electrolytes also have influences on the enthalpy value in the 6-APA crystallization process. In the sodium sulfate aqueous system. a higher concentration of sodium sulfate resulted in a lower value of enthalpy or crystallization released; however, in the ammonium sulfate system. the enthalpy of crystallization decreased when the (NH(4))(2)SO(4) concentration was lower than 0.5 mol.dm(-3) and then increased thereafter.