The purpose of this study was to investigate college students' understanding of the thermodynamic aspects of the dissolution of solids and gases. The subjects were 34 juniors from the Seoul area who answered questionnaires composed of six items which asked the directions and reasons for the changes in enthalpy, entropy, and the solubility by temperature for the dissolution of solid sodium chloride and gaseous carbon dioxide into water. The results showed that the students' understanding of the enthalpy change of dissolution was poor: many students answered that the dissolution of solids is an exothennic process because the dissolution occurs when the solute-solvent interaction is greater thati the solute-solute interaction; the students also thought that the enthalpy should be reduced for spontaneous dissolution because the spontaneity depends on the enthalpy change only. For the entropy change, the students' understanding was better and they explained it according to the meaning of disorder. For the tempemture dependence of solubility, most students answered correctly regarding the direction, but only 25% of them explained the reason accounting for the enthalpy change. Many students who answered incorrectly on the enthalpy change could not explain the reason why.