Surface state models of tribocharging have been successful in explaining many features of the charging of two-component electrophotographic developers. In this paper a new approach for constructing triboelectric series is presented based on surface state models. This method of analysis has several advantages: it requires only limited experimental data, allows the construction of triboelectric series, and makes it possible to separate the effects of the concentration of charging sites and differences in work function on charging. The method was used to analyze tribocharging data recently published by K.-Y. Law and co-workers. Both the work functions and charging site densities of toners treated with various silicas and ionic charge control agents are shown to depend on the surface composition of the toner. Surface treatment with silica causes the work function to increase, shifting the treated toner lower in the triboelectric series. The greater the surface area of the silica, the greater the shift. Hydrophobic silicas cause a greater shift than hydrophilic silicas. For toner surface treated with a set of negative charge control agents (3,5-di-tbutylsalicylic acid and its salts with Li, Na, K, and Rb) it was found that, with the exception of Cs and H, the binding energy of the ions decreases as the ions become heavier, while the concentration of charging sites increases. The results show that the high density of states limit does not apply to all of these materials.