Experimental results on the coagulation of fine coal particles in water (a case of natural hydrophobicity) and silica in dodecylamine solution (chemically induced hydrophobicity) are presented, Different behavior was observed depending whether or not the slurry was degassed, After degassing, the apparent size of both coal and silica flocs became smaller, which was attributed to removal of gas nuclei and consequent reduction in the interparticle attractive force, The effect on settling, however, was different for both : the settling rate of silica decreased while for coal it increased. This was modeled in terms of the relative effect of size and density of coal and silica particles with and without gas nuclei, Apart from practical consequences in fine particle processing, a fundamental one is that to evaluate true hydrophobic coagulation or to measure the true hydrophobic force, gas nuclei have to be eliminated.