Bubble-top jet flows were investigated using high-speed photography and laser PIV (particle image velocimetry) technology. The bubble-top jet flow formation process was experimentally observed and described in detail. Pre-jet flows were also observed in advance of the jet flow formation in the early stage of bubble growth. The developed jet flow structure was divided into four different regions. In the pumped region, the hot liquid was pumped up from wire surface. The pumping effect was an important behavior of the jet flow, which caused highly efficient single-phase heat transfer near the bubble bottom, and also caused strong interactions between neighboring bubbles. The interaction and self-organization among the jet flows produced a relatively regular flow field. Bubble size greatly influenced the jet flow intensity and increased the complexity of the jet flow structure. Multi-jet flows were observed on larger bubbles. The experimental results suggest that the jet flows were induced by surface tension gradients or strong thermal non-equilibrium around a bubble rather than the evaporation and condensation within the bubble. (C) 2004 Elsevier Ltd. All rights reserved.