It is an industrially important problem to quantitatively evaluate the charging characteristic of powder. However, the electrostatic phenomenon of powder charging is very complicated, since there are complex phenomena resulting from electrical and physical properties of powder and operational environment conditions. Therefore, the control is presently carried out in an empirical manner.In this work, an experimentation was carried out using a ferrite powder coated with silicone resin to examine the effect of the operational environment humidity on the charging characteristics of the powder as one of the factors that controls the electrostatic phenomenon. Tribo-charging of the sample powder in a rotating vessel with conductive nickel plating was measured. The sample powder was then placed with the earth in a desiccator under 10% (dry powder) or above 90% (wet powder) relative humidity for 12 h in order to attain equilibrium with the medium humidity. The absolute value of the saturated specific tribo-charge of the powder decreased with increasing humidity, and it was found that the charge of the dry powder was higher than that of the wet powder. Furthermore, there was a good correlation between saturated specific tribo-charge and the amount of water adsorbed by the powder. It was also found that the time constant for charge relaxation decreased with the water adsorption. This fact indicates that the adsorption hysteresis of the powder gives large effect in the charging characteristic of powder. (C) 2003 Elsevier B.V All rights reserved.