This study uses a one-dimensional equation system and experimental techniques to provide a comprehensive description of vertical gas-solid two-phase flow. The results from non-accelerating flow experiments conducted with a riser tube of bore 192 mm and height 16.2 m using spherical glass beads of average diameter 64 mu m are presented. The solids volume fraction, which was measured directly using quick-closing valves, was less than 0.01 in all cases. The frictional pressure drop was recognised to be an important component of the total pressure gradient in the riser. At low gas velocities, negative frictional pressure gradients occurred. The solids friction factor was found to be constant at high solids velocities and decrease to negative values as the solids velocity was reduced. The slip velocity was found to be always greater than the single-particle terminal velocity and to increase with decreasing gas velocity or increasing solids mass flux. This is different to that which has usually been reported in Literature, and is thought to be due to the large diameter of riser used in this study. In addition, the slip velocity increased (independently of solids mass flux) with increasing solids concentration.