The surface density and electrolyte concentration dependences of the pK(a) of CH3(CH2)(21)NH3+ at the air/ water interface are obtained using the method of second harmonic generation. If it is assumed that the amphiphiles are uniformly distributed at the interface, the application of the Gouy-Chapman model yields pK(a) values ranging from 10.2 to 8.2 as the interface density varies from 100 to 22 Angstrom(2)/molecule. By accounting for the inhomogeneity of the interface, i.e., there are amphiphiles islands of "liquid phase" density immersed ill a "gas phase" of very low-density amphiphiles, a single pK(a), of 10.1 +/- 0.2 is obtained at all interface densities. The acidity of the long chain amine is enhanced by 3 times at the air/water interface compared with the bulk aqueous solution.