A mechanistic study is presented on the partition behavior of mixtures of particles in interfacial partitioning with liquid two-phase systems. A model is developed based on a mechanism of competitive adsorption of particles at the liquid-liquid interface. In this model, it is assumed that partitioning is the result of a dynamic process of continuous adsorption and desorption of particles at droplet interfaces. It is shown that under certain conditions such a process can be described by means of Langmuir-type adsorption isotherms. The model is tested with partition data of mixtures of ampicillin and phenylglycine crystals in a water/n-pentane system, which leads to a reasonable quantitative description. The results indicate that for this particular system, adsorption of crystals at the interface occurs up to amounts that are needed for a monolayer coverage. In case larger amounts of crystals are present, partitioning of particles is subject to a competition for the available interfacial area. In such a case, the kinetics of adsorption and desorption of the particles to the interface seem to differ from a situation where the interface is partly uncovered. (C) 2004 American Institute of Chemical Engineers.