The dissociation constants of the following fatty acids have been determined in water relative to acetic acid over the temperature range 0 to 60 degrees C: propionic, n-valeric, n-hexoic, iso-butyric, iso-valeric, iso-hexoic, trimethylacetic, diethylacetic. Using Harned's data for acetic acid as standard, the individual dissociation constants have been evaluated. The available data for the fatty acids are analyzed to obtain the heats of ionization, and the entropy and heat capacity changes of ionization. These quantities are discussed in relation to the structure of the acid and the orientation of water molecules by the ions. It is concluded that the ions influence the water structure in a volume round the ionic charge of radius about 5 angstrom, and that the effect of the ionic charge on the rigidity of the hydrocarbon chain and on the solvent structure both play an important part in determining the entropy and heat capacity changes.