This work focuses onto the interactions between poly(ethylene oxide) (PEO) and fatty acids, in order to set their potential role of contaminants for PEO-based retention systems. Surface tension measurements were used to investigate PEO-fatty acid systems and they made it possible to clearly point out the interactions between the polymer and the sodium octadecylcarboxylates with different degrees of unsaturation. The observed interaction seems to be dependent on the fatty acids' solubility, the increase of which leads to less pronounced phenomena, which are, in contrast, emphasized by the increase in PEO chain length.