The isolated mixing regions (IMRs) or segregated regions exist above and below the rotating impeller during mixing of high viscous fluids (both Newtonian and non-Newtonian) in agitated vessels are persistent and barriers to efficient mixing. It is essential to destroy them because they occupy considerable volume inside the mixing vessel. The segregated regions can be eliminated by higher rotational impeller speed, however high shearing rates are undesirable for shear sensitive products in biological application. This work investigates the effect of impeller speed modulation to destroy the IMRs in Newtonian fluid and non-Newtonian shear-thinning fluid. The mixing time was measured using direct visualization of an acid-base reaction with fluorescent green dye as passive tracer. Glycerine (Newtonian fluid) and carboxymethyl cellulose (CMC) solution (non-Newtonian fluid) were agitated in an unbaffled vessel with inner diameter of 0.19 m equipped with a standard six-bladed Rushton turbine impeller located at the axis of the vessel. The elliptical IMRs volume was calculated by theorem of Pappus using digital photographs taken at each interval during the mixing process. For similar operating condition, the IMRs observed in CMC solution are found to be smaller and disappear earlier. It could be probably due to fluctuating viscosity of CMC solution during impeller speed modulation.