Transitional emulsion phase inversion in nonionic-surfactantoil-water systems ( (\mathrm{nSOW}) ) is brought about by changing the surfactant's affinity for the oil and water phases. In this paper an analysis is given to explain the linearity and gradient variation of transitional inversion lines (i.e., surfactant affinity difference vs volume fraction of water) in systems containing distributed nonionic surfactants. A simple surfactant partitioning model is derived which can be applied to isothermal transitional inversions where the surfactant's affinity is altered by changing the ratio of a lipophilic and a hydrophilic surfactant in the system. The model describes the partitioning of surfactant between oil, water, and a pseudosurfactant phase, which represents surfactant at oil-water interfaces. The surfactants used in this study had a distribution of chain lengths; this paper discusses when surfactants of this type may be regarded as single components. In these cases lumped parameters can be used to describe the surfactant's partitioning between the three phases. The model is applied to a number of nSOW systems and the effect of changing surfactant concentration is examined. A method of obtaining CMCs for mixed lipophilic surfactants is indicated. 1994 Academic Press. Inc.