A dynamic technique, using radioactivity as a means of detection, makes it possible to measure the partial pressures of highly polar compounds in dilute aqueous solution. The results can be expressed in terms of the dimensionless distribution coefficient for transfer of a compound from dilute aqueous solution to the vapor phase. For acetic acid this coefficient is 1.1 X 10(-5), for acetamide 7.6 X 10(-8), for N-methylacetamide 4.1 X 10(-8), and for N,N-dimethylacetamide 5.4 X 10(-7). Thus acetamide is much more strongly solvated than the uncharged acetic acid molecule. The results suggest: (1) that the peptide bond represents an extreme among uncharged functional groups in the degree to which it is stabilized by solvent water; (2) that the very great hydrophilic character of the peptide bond may be associated mainly with hydrogen bonding of the solvent to the carbonyl oxygen atom (rather than the N-H group); and (3) that the observed equilibria of biosynthesis and hydrolysis of peptide bonds in aqueous solution are largely determined by differences between reactants and products in their free energies of solvation. It is anticipated that where "bound" water is found in proteins, it will often be found to be associated with peptide bonds, and will tend to be associated with the C-O group rather than with the N-H group.