Recent data for some toners and carriers have shown that the tribo value, q/m, is independent of the toner concentration. This is contrary to the conventional response in which the tribo value decreases as the toner concentration is increased. Because conventional models seemingly do not predict this behavior, new models have often been proposed to explain this effect. In this work we examine two toner–carrier pairs for which the tribo values appear to be independent of toner concentration. This result is obtained when separate developers are made at different toner concentrations. By changing the toner concentration using a technique that preserves the surface properties of the toner and carrier, we show that the conventional m/q:toner-concentration dependence is obtained and is consistent with the Gutman–Hartmann (G–H) model. The unusual behavior in which the tribo value appears to be independent of the toner concentration is a result of changes in the toner and carrier surface composition when developers are prepared and mixed at different toner concentrations. Surface measurements of toner and carrier confirm toner-concentration-dependent surface modifications. We also compare the G–H model to other published data showing m/q independent of toner concentration. In the latter case, there are no anomalous surface effects. The data were taken over a wide range of toner concentration: in the low toner concentration regime, the dependence of m/q is small. We found that the G–H model is consistent with the data at both low and high toner concentration due to a slight curvature in the model that is difficult to observe experimentally unless the data are taken over a wide range of toner concentration.