In this paper the influence of surface-active trace impurities in aqueous oligoethylene glycol monooctyl ether solutions on their adsorption behavior at the air/water interface is considered. The equilibrium surface tension ( e ) vs logarithm of concentration (ln c) isotherms of surfactants when used "as received" usually reveal a broad concentration range characterized by a constant slope, d e /d ln c. This linear dependence of the equilibrium surface tension on the logarithm of the surfactant concentration is observed for various surfactants at various fluid interfaces, which contradicts the fundamental Gibbs equation given in Eq. [1]. The same behavior was also observed for aqueous solutions of chemically well-defined oligoethylene glycol monooctyl ethers. In this case the paradoxical behavior did vanish after purification of aqueous solutions until the state of "surface-chemical purity" was reached using an apparatus for programmed high-performance purification of surfactant solutions. The gas chromatographic analyses of the original samples used showed that they contained impurities of different chemical structure and surface activity. Analysis of material aspirated from the surface of these alcohol ethoxylate solutions indicates that these trace impurities are enriched or impoverished at the solution's surface. A clear correlation between the surface activity parameter and the enrichment/ impoverishment was found. Thus, these findings prove that the paradoxical adsorption behavior (d e /d ln c β«Ψβ¬ constant) of the oligoethylene glycol monooctyl ethers is caused by surface-active trace impurities. These results support the hypotheses suggested by K. Lunkenheimer and Ch. Wedler [Tenside, Surfactants, Detergents 30, 342 (1993)] that generally it is the effect of surface-active trace impurities that gives rise to paradoxical adsorption behavior with surfactant systems at fluid interfaces.