This report is a study of the kinetics of contact charging resulting from the rolling of millimeter-sized metallic spheres on flat surfaces of polystyrene (PS). Charging was studied (i) for different values of the relative humidity (RH) of the air in contact with the system, (ii) in acidic and basic atmospheres, and (iii) for a series of PS surfaces oxidized to different degrees. The rates of charging increased with increasing RH; they were higher in basic atmospheres than in neutral or acidic ones and correlated with the numbers of hydrophilic groups on the surface of the polymer. These findings are consistent with a mechanism that implicates a thin film of water on the surface of PS as an important element of tribocharging in this system.