Adsorption data in the form of surface tension vs hulk concentration isotherms having strong changes in the: slope have often been discussed in the literature in terms of phase transitions in the adsorption layer. It will be shown thermodynamically that these apparent kink points in the isotherms are unreal. A phase transition would cause a completely opposite change in the isotherm. Experimental surface tension data for sodium octanoate and dodecyl ammonium chloride obtained from the literature are reinterpreted on the basis of the Frumkin and reorientation isotherms.