This paper is concerned with semi-batch foams generated by injecting gas bubbles in a vertical column containing a liquid phase at rest. Its aim is to better understand the physical mechanisms responsible for foam formation at the liquid free surface and to predict the superficial gas velocity for onset of foaming. The model for predicting the onset of foaming is derived from the one-dimensional drift-flux model for gravity driven flow in the absence of wall shear. The analysis is based on experimental data reported in the literature and covers a wide range of physico-chemical properties, bubble sizes and shapes, and flow regimes. It identifies the inhibition of coalescence between rising bubbles and bubbles at rest at the free surface as a key mechanism responsible for the onset of foaming. A semi-empirical correlation for high viscosity fluids has been developed and good agreement with experimental data is found. (C) 2003 Elsevier B.V. All rights reserved.