This study considers the tribo-electric charging of materials during the dilute phase pneumatic conveying of powders. The main objective of the work is to develop a phenomenological model capable of describing the spatio-temporal evolution of charges of both the powders and pipe wall. The model is based on a second order kinetic for the charge transfer between the two materials (i.e. the elementary mechanism of charge transfer is considered to be the binary particle wall collisions). The results show that the excess or lack of chargeability of the wall is the key parameter regarding the charge acquired by the particles. For wall chargeability levels less than that of the powder (excess ratios smaller than unity) the pipe wall becomes almost saturated over time so that the accumulated charge of the particles tends to an asymptotic limit imposed by the chargeability of the wall. For excess ratios greater than one, the particle charge is no longer limited by the charge of the wall but by the intrinsic chargeability of the particles. The accumulated charge of particles varies linearly with time and could potentially tend to saturation. In addition, for initial excess ratios greater than 3, the particle charge varies linearly with time. Finally, a comparison between the experimental and calculated data was carried out and permitted the validation of the model. (C) 2011 Elsevier Ltd. All rights reserved.