The adhesion force between a bubble and a particle has been measured in relation to flotation froth formation. A novel experimental apparatus has been used for measuring adhesive forces in the micro-Newton range (10(-6) N), whilst providing advanced optical monitoring of the bubble-particle interaction. We focused our study on two important parameters that affect the adhesion force in flotation froths, namely the degree of particle hydrophobicity and the presence of surfactants in solution. We used glass spheres of four different hydrophocities and detached them from bubbles in pure water. We experimentally verified that the more hydrophobic the particle, the higher the adhesion force. The adhesion force was affected to a great extent by the presence of surfactants in water, due to their effect on surface tension and particle hydrophobicity. As a result, the adhesion force lowered with increasing surfactant concentration. Our results provide experimental evidence of the effect of natural organic matter on particle-bubble adhesion forces and, as a result, on flotation processes used in drinking-water treatment and the stability of flotation froths.