This study reports on the unaerated power number (Np) and the ratio of aerated to unaerated power consumption (P-G/P) for three disc type impellers in water and highly viscous Newtonian and non-Newtonian liquids. Modified disc type impellers (i.e. CBDT-concave bladed disc turbine and Scaba 6SRGT) were found to give lower Np and higher P-G/P values than the flat bladed disc turbine or Rushton turbine (DT), in all liquids. Higher P-G/P values are generally assumed to imply superior mass transfer potential of a given impeller. However, an examination of gas hold-up (which is closely linked to mass transfer) generated by DT, CBDT and Scaba 6SRGT, suggests that this is not necessarily true and further investigations may be necessary.