Residence time distribution (RTD) measurements performed in a small laboratory flotation column using an electrolyte tracer, showed that, without air injection, cellulose fibres induced a sizeable decrease in tracer average residence time, owing to the formation of stagnant zones. Bubble size, fibre flocculation level and gas hold-up were recorded during flotation experiments with different fibre and surfactant concentrations: a slight decrease in bubble size, fibre flocculation level and increased gas hold-up were measured after surfactant addition. The increase of cellulose fibres concentration promoted bubble coalescence, pulp flocculation and gas hold-up. Experimental data interpretation with the drift flux model demonstrated that fibre flocculation depressed gas hold-up by raising the bubble rise velocity. Moreover, correlations between fibre flocculation and their transport during the flotation process showed that pulp flocculation also promoted bubble entrapment in fibre flocs and their subsequent entrainment by air bubbles into the froth. (C) 2011 Elsevier B.V. All rights reserved.