Maleic acid mono[2-(4-akylpiperazinyl)ethyl esters] (C-8-C-13) and their precursors, 1-alkyl-4-(2-hydroxyethyl)piperazines, are synthesized and the pK(a) values of the surfactants are determined by potentiometric titration. The different ionic forms of the surfactant and their content in the solution at definite pH values are determined within the range pH 0.5-10. At pH = 6.2 the predominant component in the solution has a betaine-like structure. As this form is expected to have the highest surface activity and the concentrations of other ionic species are low in the bulk phase, this pH is selected to determine. the adsorption parameters of the homologous series. A special method to determine equilibrium values of the surface tension is used in cases of low surfactant concentrations. To determine the adsorption parameters from sigma-log C curves of the aqueous surfactant solutions a Frumkin-Damaskin isotherm is used. The cmc (critical micelle concentration), the interfacial activity, and the interaction parameter show a significant even/odd alternation with the number of alkyl groups of these surfactants. in accordance with the alternation of the melting points.