Equilibrium surface tension (sigma(e),) versus concentration isotherms of surface-chemically pure aqueous solutions of the homologous series of N-n-alkyl-4'-(dimethylamino)-stilbazium bromides ("hemicyanines") were measured at 295 K. The adsorption parameters of saturation adsorption and standard free energy of adsorption of the hemicyanines were determined from the evaluation of the c, vs c isotherms using a two-state approach to the surface equation of state. The adsorption parameters reveal a very pronounced phenomenon of alternation (even/odd effect), which has so far not been found to a large extent such as this. Thus, the cross-sectional areas of the odd members are almost twice those of the related even members. Surface activity of the odd members is stronger than that of the even members. UV-vis investigations showed that there is no indication of aggregate formation in the adsorption layer. The thermodynamic results give evidence for distinct differences between the surface conformations of the even- and the odd-chain hemicyanine dyes although the reasons for this are not known.