Charge control agents (CCA's) are frequently added to toners to create a desired charge level and polarity. Triboelectric charging tests on model, CCA-surface-treated toners and carriers indicate that CCA transfer between toner particles and carrier beads can be a controlling factor for the magnitude and polarity of toner charge — in extreme cases, toner polarity can even be reversed as a result of CCA transfer. The tests also indicate that the transfer process (and hence the final charging properties of a toner) can be significantly affected by the concentration of the toner in a xerographic developer.