The adsorption behavior of maleic acid mono[2-(4-alkylpiperazinyl)ethyl esters] (C-12 and C-13) is characterized in a wide range of pH values (0.5-10.5) by measuring the surface tension and calculating the adsorption parameters of a Frumkin-Damaskin isotherm. In dependence on the pH, four different species of the surfactants exist. The adsorption properties of these species are characterized at pH values, at which one of the species adsorbs preferentially. In general, the surface activity increases with the pH. At pH = 0.5 the concentration of Cl- was so high that the surface activity of the species, which adsorbs favorably at this pH values, decreases. To compare the adsorption parameters of this species, the adsorption behavior in the range pH 1-6.2 was investigated in the presence of 3 M/dm(3) of NaCl. At pH values of two different coexisting species, the adsorption is described by a generalized Frumkin-Damaskin isotherm. Thereby, an additional interaction parameter along with the adsorption parameters of the individual species is used, which quantifies the interaction between the different coadsorbing species at the interface. This additional interaction turns out to be weak, but significant. As the consequence of the interaction between the molecules of the individual components, the relative surface concentration of a component can be increased. In some cases, however, the surface concentration of the less surface active component is reduced by the coadsorption of stronger interacting molecules and/or higher surface activity.