In this paper the mechanism of discharge of fine powders from hoppers is examined. Gas pressure measurements were taken in the proximity of the hopper outlet, mainly along the axis, while discharging fine powders. Different types of pressure traces were detected depending on the solid properties. A simple model was developed to account for the main features of pressure profiles. The main hypothesis relies on the presence of a surface near the hopper outlet which defines a discontinuous change in the voidage of the discharging solids. From the fluid dynamic point of view, the solid above this surface bl:haves as a permeable compact; the solid below, instead, can be considered as a moving suspension of solids in gas. This region appears to be the major responsible of the pressure gradients inside the hopper. A good agreement between model and results indicates that, in spite of its simplicity, the models accounts for the main phenomena involved.