Mixtures of sodium monodecyl phosphate (NaDeP) and poly(ethylene oxide) (PEG) have been investigated conductimetrically in 0.05 M succinate buffer, pH 5.20. The conductivity vs NaDeP concentration profiles do not show the classical behavior of PEO-sodium dodecyl sulfate (SDS) mixtures. In the presence of different concentrations of PEG, the profiles exhibit only one discontinued point, whose SDS concentration values are similar to the NaDeP critical aggregation concentration. It was observed also that this conductivity breakpoint is not sensitive to variation in the polymer concentration and that it is independent of polymer molecular weight. However, the conductivity profiles of mixtures of NaDeP and SDS demonstrate behavior similar to that of SDS alone. For these mixtures, the evidence of binding, absent in the presence of NaDeP alone, is interpreted by assuming that it is solely driven by the SDS.