Applying the two- fluid concept. momentum transfer by solids in gas-solids flow through risers is described in terms of solids-phase shear viscosity. Viscosities for glass- and FCC-particles are estimated from literature data on radial profiles of solids concentration and solids velocity in two CFB-risers. Results are compared with simple kinetic theory of granular flow, leading to unreasonable high values of granular temperature. In contrast to kinetic theory, results also indicate a decrease of viscosity with increasing shear rate. Therefore, it is proposed to assume particle clusters as momentum transferring structures. Qualitative analysis of viscosity results by adopting kinetic theory to clusters suggests cluster size decrease with increasing shear rate. These results indicate importance of local shear rate on structure of aggregative solids Row.