Bubble formation by releasing gas-(CO2-)supersaturated slurry of fine particles (d(50) < 5 mu m) from a pressure chamber was investigated. Gas holdup, defined as the percent volume of gas phase in the form of bubbles in the suspension released from the pressure chamber, was used to evaluate bubble formation. Compared to the result without solids, gas holdup decreased in the presence of hydrophilic silica; a small quantity of naturally hydrophobic coal increased gas holdup by increasing the number of sas nuclei and nucleation sites, while the presence of a large quantity of coal reduced the gas holdup, due to the coalescence of bubbles induced by the hydrophobic particles. For silica particles of chemically induced hydrophobicity in dodecylamine solutions, gas holdup increased sharply with solid content, reaching a maximum at similar to 2% w/w solids. The results suggest that for hydrophobic particles to act as bubble nucleation sites, macroscopic contact angles do not have to be greater than 90 degrees under dynamic conditions.