We present an experimental study on the bubble-slug flow pattern transition which takes place in two-phase flows in microgravity related conditions. Two different sets of experiments were performed on ground with air/water and air/ethanol mixtures in a minichannel with an internal diameter of 1 mm. We address questions regarding the existence of a critical void fraction for the bubble-slug transition to occur. Experimental data are compared to the drift-flux and Suratman models predictions. We obtain results on the extension of applicability of these models for different boundary conditions. (C) 2013 Elsevier Ltd. All rights reserved.