The charging behavior of toner particles in two-component electrophotographic developers is consistent with a thermodynamic model of charging. This model, often referred to as the “surface state model,” provides a basis for determining triboelectric series. It can also be used to predict the magnitude of charge exchanged between particles provided the model parameters are known for each material. The surface state model was used in conjunction with triboelectric measurements to demonstrate this capability. Model parameters were determined experimentally from two-component mixtures made from several carriers and toners. The model was then used to calculate charging in three-component mixtures of the same toners and carriers. Excellent agreement was found between calculated and experimental charge-to-mass ratios. Multicomponent developer mixtures may arise in many practical situations, such as the introduction of new toner formulations into an existing product, cross-contamination of developers in multicolor copiers, and changes in the charging properties of toners and carriers resulting from developer aging in service. The presence of more than two components in a developer often causes developers to fail. The ability to predict tribocharging behavior of individual components in a multicomponent developer mixture is very useful in explaining observed developer failures and identifying potential problems. Several applications of surface state models in practical situations are discussed.