Batch mixing of rheological complex fluids using helical ribbon and helical ribbon screw impellers Ras investigated in the laminar mixing region. Eight models and one polysaccharide fermentation broth (gellan) were used. These fluids exhibited different rheological properties : Newtonian viscosity, viscoelasticity, and pseudoplasticity. The Ostwald-de Waele model (power law) described adequately the shear viscosity of pseudoplastic fluids, including the gellan broth (n ranged from 0.14 to 0.65). Power consumption decreased as pseudoplasticity increased and a model was developed to predict the deviations from Newtonian power input due to pseudoplasticity. A dimensionless, unique representation of the power data using the predictions of the model, showed a shift of the upper limit for the laminar mixing region towards higher values of Re (Re approximate to 100). The model predictions were found to be in good agreement with those obtained by using the classical approach of Metzner and Otto(1). The constant of proportionality between the average shear rate and the impeller rotational speed, K-s, was found to be a function of the power law index, especially for high shear thinning fluids. It was found that power consumption increased as the impeller blade pitch decreased or as the impeller blade width increased. Both impeller pitch and blade width do not have significant influence on K-s.