Acid/base interactions have been studied by means of surface tension, calorimetric measurements, and Langmuir films. When a fatty amine is combined with hexanoic acid, the surface activity of the corresponding complex is increased as the base is shifted from butylamine to octylamine. The complexation in the pure system hexylamine/hexanoic acid is accompanied by an exothermic enthalpy change of -45 kJ/mol. The Langmuir study reveals that complexes of butylamine/hexylamine and stearic acid are water soluble, while octylamine was incorporated into the monolayer of stearic acid. Monolayers of arachidic acid took up butyl- and hexylamine with retained stability. When combined with octylamine, the arachidic acid monolayers became unstable due to nucleation processes.