The use of helical ribbon agitators to mix viscous non-Newtonian fluids has been investigated. A generalized model, based on an appropriate definition of effective viscosity, is proposed to predict power consumption. This model is most successful with fluids that do not have a high degree of elasticity.It was found that the efficiency of mixing of pseudoplastic fluids was about half of that of Newtonian fluids in the same mixer, while the efficiency of mixing viscoelastic fluids was still lower and approximately independent of the mixer geometry. Blade width was the primary variable affecting the mixing efficiency on inelastic fluids. SCOPEThe mixing of very viscous fluids can be efficiently accomplished with a helical ribbon agitator (HRA). A model to predict the power consumption of the HRA mixing Newtonian fluids has been reported by Patterson et al. (1979); however, very viscous fluids are often non-Newtonian or viscoelastic in nature.