The goal of this study is to present new experimental data on the effect of the bubble approach velocity and liquid viscosity on pairwise bubble coalescence. Measurements were performed to investigate the dynamics of bubble coalescence under well-defined laboratory conditions. Air and pure aqueous solutions of alginate (no surfactants) were the phases. The bubbles were formed from two vertical capillaries, under constant flow conditions. The coalescence process was recorded with a fast video and then image analysed, to evaluate the bubble growth, bubble expansion rate, the first bubble touch, bubble contact time T, bubble coalescence efficiency E. Three control parameters were tested. The bubble size D on contact was set by the spacing between the capillaries (D=1 and 1.5 mm). The bubble approach velocity V was controlled by the gas input (V=0.031-39 mm/s). The liquid viscosity mu was adjusted by the alginate content (mu=1.2-10.3 mPa s). It was found that the bubble contact time T monotonously increases with the liquid viscosity, passing through three stages: initial rise (low mu, coalescence), then jump (intermediate mu, transient region), and finally forming a plateau (high mu non-coalescence). In the coalescent regime, the bubble contact time monotonously decreases with the bubble approach velocity obeying a power law, which for a typical 1 mm bubble reads: T similar to V-0.85. The difference in the bubble size did not change the coalescence pattern qualitatively, but only quantitatively in a modest manner. (C) 2015 Elsevier Ltd. All rights reserved.