Turbulent flow and bubble coalescence in water at the leeward side of a baffle in a stirred tank reactor have been studied with PIV and a high-speed CCD camera. A large stationary vortex was revealed at the leeward side of the baffle. At impeller speeds of 400-600 RPM, the maximum tangential velocity at the vortex was proportional to the impeller tip speed (u(theta) approximate to 0.27 U-tip), giving a pressure difference in the vortex of as much as a few hundred pascal. The main mechanism for coalescence in this area was trapping of bubbles in the vortex, increasing the local hold-up and coalescence probability by forcing the bubbles together. The force on the bubbles resulting from the local pressure gradient makes the bubbles move fast to the centre, within 10-20 ms. The coalescence efficiency was very high and more than 85% of the coalescence occurred within 2 ms. Bouncing of bubbles was mainly seen for large bubbles moving up in the centre of the vortex. The trapping of the bubbles was modeled successfully by a drag coefficient model corrected for nonspherical bubbles and turbulent continuous phase. (C) 2007 American Institute of Chemical Engineers.