The mechanism of humidity effect on the charging of xerographic toner is studied using model toners prepared by solution coating charge control additives (CCAs) lithium 3,5-di-t-butysalicylate (LitBSA) and cesium 3,5-di-t-butylsalicylate (CstBSA) onto the surface of 9 μm styrene-butadiene toner at a concentration of 4 μg of CCA per gram of toner. These toners were shown to acquire negative charges against polymer-coated metal beads largely by an ion-transfer mechanism. Investigations of the charging of these toners at 20 and 80% relative humidity (RH) show that there is a ∼29% decrease in toner tribo (toner charge), from −80 to −57 μC/g for the LitBSA toner as the ambient RH (relative humidity) increases from 20 to 80% during charging. On the other hand, the charging of the CstBSA toner is RH insensitive; the toner tribo remains at 29.5 ± 1.5 μC/g under identical conditions. The surfaces of the recovered metal beads (after tribo measurements) were analyzed by the time-of-flight secondary ion mass spectrometry technique. While Li⁺ and Cs⁺ are expectedly observed on the respective surfaces of the recovered metal beads due to the ion-transfer mechanism, we are surprised to find that the amount of ion that is transferred is insensitive to humidity changes for both toners, despite the fact that we observe an RH effect on toner charging for the LitBSA toner. In the negative mass spectral experiments, we observe an RH effect on the surface chemistry of the recovered metal beads. For beads that were charged with the LitBSA toner at 80% RH, we detected a small quantity of tBSA⁻ at m/z 249. The 249 peak is not observed for beads that are contacted with the LitBSA toner at 20% RH, nor with the CstBSA toner at both high and low RHs. Based on the RH effects on toner charging and the surface chemistry, we suggest that the humidity effect on the charging of the LitBSA toner originates from a charge-recombination process between Li⁺ and tBSA^- at 80% RH. The critical roles of the “water bridge” and the electric field developed between the charged toner and the metal beads on the humidity effect are discussed.