The aqueous mixtures of two nonionic surfactants, pentaethyleneglycol monohexyl ether (C6E5) and hexyl dimethyl phosphine oxide (C6DMPO), were investigated by the pulsed-gradient stimulated-echo NMR technique. Quite unexpectedly, the results show that the mixture behavior significantly deviates from ideality. Particularly, analysis of the data indicates that, in the mixed aggregates, C6E5 molecules prefer to be surrounded by other C6E5 molecules, forming domains of hydrated ethoxilic chains on the micellar surface. Molecular reasons for the segregative tendency of ethoxylated surfactants and its applicative implications in formulation technology are discussed.