Effects of liquid viscosity on coalescence of a bubble upon impact with a free surface were studied both experimentally and numerically. Experiments were conducted in a pool filled with silicone oil by generating a nitrogen gas bubble and observing it with a high-speed video camera. Full Navier-Stokes equations were solved with two level set functions for a bubble and a free surface. As a result, the Weber number was the most important parameter determining the coalescence time in low viscosity liquids. in contrast, the coalescence time in a high viscosity liquid was much greater than that of a low viscosity liquid with the same Weber number. In addition, the thresholds between bubble coalescence and bouncing were affected by liquid viscosity. For the differences in coalescence time, foam was observed at the free surface in the case of high viscosity case only the difference of liquid viscosity in the non-polar liquid. By examining the pressure distribution of the liquid film between the bubble and the free surface and the downward liquid flow, we conclude that not only the liquid film but also the liquid flow field underneath the bubble was important for bubble coalescence or bouncing. (c) 2005 Elsevier Ltd. All rights reserved.