Abstract The effects of water and heavy water on conformational equilibria of fluoroacetone have been investigated via Raman spectroscopy. Additional Raman bands have been observed in the CF stretching and the CCC symmetric stretching regions for the aqueous solutions. Based on enthalpy and volume differences between the conformers, these bands are assigned to the syn conformer which has hydrogen bonds between the fluorine atom and water molecules ( syn′ conformer). The number of H 2O molecules binding to the syn′ conformer is estimated to be 2.4 from the concentration dependence of the spectrum. The enthalpy and the volume differences between the cis and syn conformers in the aqueous solutions show anomalous values in comparison with those in organic solvents. We discuss these thermodynamic behaviors from the viewpoint of the hydration structures of fluoroacetone.