In this work, the effect of the vessel bottom roughness on the suspension of solid particles in stirred tank reactors is investigated. The experiments were performed in a baffled vessel, which was mechanically stirred with a 45 degrees pitched blade turbine. In order to evaluate the influence of the bottom roughness on particle suspension, four bottoms of different roughness and 8 different sets of spherical particles were used. The density of the solid particles, rho(s), ranged from 2500 kg m(-3) to 8743 kg m(-3) and they were characterized by narrow size distributions with a mean diameter, d(p), from 128 mu m up to 1850 mu m. Measurement of the minimum impeller speed for 'just complete suspension', N-js, showed that the roughness of the bottom had a significant influence. The precise effect depends on the particle size compared to the size of the roughness elements and to the Kolmogoroff microscale, lambda(K).