The charge-admix-exchange process happening in the xerographic developer between aged and fresh toner, as well as carrier particles carrying different electrostatic charges, is a complex and not well-understood process from a theoretical point of view. Fast electrostatic charging of fresh toner particles to a desired steady state in a developer is very important to achieve high quality prints, especially in the case of high speed printing devices. One common way that formulators try to improve toner admix is by adjusting the external additive package [1-3].The goal of this study was to gain a better understanding of how different additive morphology and surface treatment influence admix. More specifically, we investigated what happens with the electrostatic charge of toner when aged and fresh developers are mixed in equal proportions and the mixture exercised over a short time period.Results of the study of a simple model developer containing a positively charging toner, a single fumed silica surface treated with a silane containing an amino group, and a silicone coated ferrite carrier, will be presented. The collected data indicate that the carrier's ability to gain electrostatic charge quickly decreases as a result of the transfer of silica particles or molecules of the silica treating agent to the carrier's surface. This leads to an unfavorable performance in the admix test.In the second part of the work, the effect of different types of spacer additives on carrier poisoning and admix was studied. The obtained results show that the formulations containing spacer particles have more stable charge vs. exercise time and better admix characteristics than the formulation containing only one silica additive.