This study aims to investigate the breakup of slender bubbles in non-Newtonian fluids in microfluidic flow-focusing devices using a high-speed camera and a microparticle image velocimetry (micro-PIV) system. Experiments were conducted in 400- and 600-mu m square microchannels. The variation of the minimum width of gaseous thread with the remaining time before pinch-off could be scaled as a power-law relationship with an exponent less than 1/3, obtained for the pinch-off of bubbles in Newtonian fluids. The velocity field and spatial viscosity distribution in the liquid phase around the gaseous thread were determined by micro-PIV to understand the bubble breakup mechanism. A scaling law was proposed to describe the size of bubbles generated in these non-Newtonian fluids at microscale. The results revealed that the rheological properties of the continuous phase affect significantly the bubble breakup in such microdevices. (c) 2012 American Institute of Chemical Engineers AIChE J,, 2012