We carry out experiments to investigate the triboelectric charging due only to particle–particle collisions as a function of humidity. At low humidity, we find that large particles tend to charge positive and small particles tend to charge negative, in agreement with previous studies. However, at high humidity, we find no significant particle-size dependence for the particle charging. To explain these results, we apply a theoretical model based on charge carriers trapped in nonequilibrium surface states characteristic of electrically insulating materials. Monte Carlo simulations show that collisions between particles enable the charge carriers to reach lower energy states on other particles. These nonequilibrium dynamics lead to an accumulation of charge carriers on small particles, and if the charge carriers are negative (electrons or negative ions), the small particles would tend to charge negatively. We propose that humidity leads to conductive layers on the surface of particles that act as a sink for charge carriers and thus reduce the particle-size-dependent charging that follows from the presence of the charge carriers in nonequilibrium states.