Knowledge of nonideal behavior of surfactant mixtures of nonionic surfactants at air/aqueous solution interface is important in order to obtain surfactant mixtures with improved properties. In this work interfacial behavior of Brij S20/P407 and Triton X-100/P407 surfactant mixtures was investigated. The nonideal behavior was elucidated by extending Clint's theory and by determination of the ideal maximum surface excess concentration values. Rosen's model for mixed monolayers was used to investigate interactions between different surfactants at the interface. Synergism was noticed in Brij S20/poloxamer 407 mixtures. However, very different interfacial behavior was observed for Triton X-100/poloxamer 407 systems. (C) 2018 Published by Elsevier B.V.