Concentration distributions of solid particles transported by pseudoplastic fluids in laminar regime were determined experimentally. Practically all the distributions show minimum concentration of solids near the centre of the pipe and higher toward the walls and even, for some conditions, the highest values near the top of the pipe. This behaviour of the concentration distribution was explained when diffusive fluxes of the solid particles due to concentration gradient and viscosity gradient are considered. An existing diffusive model was modified in order to take into account negatively buoyant particles and a pseudoplastic fluid as a carrier. It was found that the head loss plays an important role in the fluxes of solid particles due to gradients of concentration and viscosity, increasing them enough to overcome the effect of gravity.