Three toner-carrier combinations were prepared by using a styrene-acryl (St-Ac) model toner and three types of ferrite carriers (having an electron accepting, a neutral, or an electron donating coating on its surface). Six types of CCAs (Charge control agent) were externally added to the interface between the toner and carrier. The amount of toner charge q/m generated in each combination that contained the same amount of CCA was measured by a blow-off method and compared with the amount of toner charge q₀/m generated in the same combination that contained no CCA. By adding six types of CCA, the difference of the amount of toner charge Δq/m (= q/m − q₀/m) increases linearly with the increase of the Electron-donating Coordinate (EDC) that is determined by the amount of blow-off charge q₀/m. Six Δq/m-EDC characteristic curves obtained by the addition of six types of CCA shift toward the positive charging region with the increase of the positive charge impartation capability of the added CCA. The zero-point-of-charge-shift (zpcs) value, which is defined as the EDC value at Δq/m = 0 in the Δq/m-EDC characteristic curve, decreased with the increase of positive charge impartation capability of the CCA. Using the above results, the role of CCA particles in the toner/carrier interface was correlated to the EDC value of the carrier from a viewpoint of electron-donating (or electron-accepting) characteristics between toner/carrier, toner/CCA and carrier/CCA interfaces.