AbstractThe activity coefficients of the n‐primary, secondary and tertiary aliphatic amines in very dilute aqueous solution were obtained from determinations of partial pressures or from solubility measurements. the activity coefficient of n‐butylamine increased 1.26 times when the ionic strength of the solution was raised from 0 to 0.75, the approximate value for sea water.Free energies of solution, hydration and vaporisation all showed regular increases with the number of carbon atoms in the molecule. the free energy of solution of primary, secondary and tertiary amines increased by 727, 700 and 610 calories respectively and the free energy of hydration by 85, 143 and 52 calories for each carbon atom. Isomeric aliphatic amines showed an increase in total free energy of hydration in the order, primary, secondary, tertiary, indicating that, as with the isomeric alcohols, the free energy of hydration increased as the molecule became more comp