This paper presents investigation on the possibility to improve the efficiency of mixing of the highly viscous Newtonian fluids in a vessel with utilization of unsteady rotating dual turbine impellers. Flow visualization experiments were used to examine the size, positions and structure of the IMR (Isolated Mixing Regions) regions as a function of Reynolds number as well as mixing time. Additionally, the effect of frequency of impellers' oscillation on mixing efficiency was examined. It was found that the use of unsteady forward-reverse mixing mode enhance the mixing efficiency in comparison to standard mixing (up to about eight times). The structure of IMR in the forward-reverse mixing is much more complicated than in standard mixing, because of liquid division into spiral-shape filaments. This had caused that dimensionless mixing time was up to about eight times shorter in comparison to standard mode. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.