The effect of humidity on the contact charge which develops between the ion-containing polymeric powders and steel beads was examined. The powders are a blend of a styrene-butyl methacrylate copolymer and minor amounts of the acid form of a partially sulfonated polystyrene ionomer or the corresponding sodium salt. At 0% relative humidity (RH), the contact charge on the blends is effectively that of the host copolymer. It rises as the humidity increases from 0 to 20-40% RH and then decreases at the higher humidities. A higher contact charge is attained with the acid form of the ionomer than with the corresponding sodium salt which in turn charges higher than the copolymer. The surface water layer thickness of the blends was measured by ellipsometry and also found to increase with humidity. The optimal charge is observed in the humidity range where about one monolayer of water adsorbs on the polymer and oxide surfaces. The decrease in charge at the higher humidities is attributed to the ineffective mixing between the powder and the beads during rolling, plus charge "run-off" due to reduction of the surface electrical resistance as detected by ac dissipation. The contact charge is attributed to the mobilization of dissociated ions (from the ionomer) in the surface water layer followed by ion transfer to the second surface during the contact.