The partitioning of ethoxylated alkylphenol surfactant species between oil and water depends on all physicochemical variables which are likely to alter the balance of affinity of the surfactant for the oil and water phases. Experimental data indicate how the logarithm of the partitioning coefficient varies with the oligomer characteristics (degree of ethoxylation, alkyl chain length), the phase nature (oil alkane carbon number, aqueous phase salinity), and cosurfactant (n-pentanol) content.