Local solids densities (concentrations) at a test cross section of a concentric annulus in the fully developed flow region were measured by using an isokinetic sampling technique during upward flow of water-feldspar slurries. As solids, 72 or 138 mu m feldspar particles at an average concentration of 1 or 2% v/v (volume of solids/volume of vessel) were used in the feed slurry. The amounts of solids in the collected slurry samples from different radial locations were determined by a gravimetric method (Ozbelge and Somer, 1988; Ozbelge and Camc iota, 2004; Camc iota, 2003). In turbulent flow regime, at relatively lower mixture velocities (slurry Reynolds number, Re m , being around 9000), radial solids concentration profiles showed a local minimum at a dimensionless radial distance (DRD) of 0.45, then increasing trends towards the inner and outer walls of the annulus were observed. As the slurry Reynolds number increased further to 19500, the solid particles gathered more at around DRD = 0.45 in the annular gap and were transported near the axis rather than near the walls. Accordingly, the two-phase pressure gradients in the test section decreased with the superficial water velocity in the 9000-19500 range of the slurry Reynolds number, most probably as a result of decreasing particle-wall interactions. In the laminar flow regime, local solids concentrations being extremely low due to the transportation difficulty of solids up to the test section at very low velocities, the two phases behaved like a single phase, and the pressure gradients increased with the Reynolds number from Re m = 2000 up to Re m = 9000. The two-phase pressure gradients increased with the increasing consistency of the feed slurry and particle size at each superficial water velocity.