Phenomena occurring in the froth zones of mechanical cells and columns during flotation have a significant effect on flotation results. For example, they affect recovery because a substantial proportion of the particles detach themselves from the bubble surface after coalescence and return to the pulp. In the framework of the Union European Impexflotcol project, coalescence and the role of one of the most common frothers, the MIBC, were studied in the laboratory, in order to understand the phenomena controlling the selectivity of column flotation. A-new equipment was set up in order to observe the consequences of one coalescence between two bubbles mineralized in the conditions of an industrial operation (almost fully loaded by particles). A selectivity in the detachment of particles during coalescence was observed in relation to their hydrophobicity. The frother is one of the factors that can favour coalescence and thus selectivity. The importance of its action in flotation froths (Klimpel and Isherwood, 1991: Int. J. Miner. Process, vol. 33, pp. 369-381) led us to study its properties. A method was developed enabling the quantification of (1) the ability of MIBC to reduce the coalescence of bubbles, and (2) the foaminess of the reagent in the presence or absence of solid particles. Foaminess measurements show that MIBC frothing properties are closely linked to mineral hydrophobicity. The presence of more hydrophobic particles (chalcopyrite) leads to greater maximum foam heights compared to those obtained with less hydrophobic particles (sphalerite) and without particles. The influence of particle size and concentration has also been shown.