The coalescing mechanism of in-line two-unequal bubbles rising in a bubble column with a non-Newtonian fluid has been experimentally studied. The non-Newtonian fluid is the 1.5 wt% polyacryamide in demineralized water and the bubble gas is air. Both qualitative flow visualization and quantitative full-field measurements using particle image analyzer have been conducted for two examples of two-unequal bubble coalescences. The instantaneous flow structure and the shear stress contour varying in different time lags are presented. As two-unequal bubbles are getting closer, the drained liquid is circulated to the back of the trailing bubble. Due to the drainage flow and the mutual interactions, the flow structure and the shear stress contour show a dramatic change from a two independent bubbles to a single bubble. Due to encounter these dynamic flow structures, the shape of the trailing bubble also has serial changes. However, the case of large trailing bubble with large momentum can affect the deformation of the small leading bubble earlier. Due to the dragging force caused by the negative pressure and the shear-thinning effect and the pushing force caused by the viscoelastic effect, the acceleration of the trailing bubble to the leading bubble is evident. It was found that the coalescence of two-unequal bubbles has higher acceleration than the coalescence of two-equal bubbles. (C) 2009 Elsevier B. V. All rights reserved.