Particle-stabilized aqueous foams are encountered in radioactive waste treatment and immobilization processes and in food, chemical, and agricultural products. The cause of foaminess in the presence of finely divided solids during boiling and in the absence of any surface-active agents is not well understood. Our research has identified at least two kinds of particles in such foaming Systems, hydrophilic (i.e., water wet) colloidal particles dispersed in the aqueous phase and biphilic particles (partially wetted by water). The biphilic particles are attached to the air-water surface. In this study we used an advanced optical technique to characterize and monitor the number of nonattached (i.e., hydrophilic) and attached (i.e., biphilic) particles at the gas-liquid surface. The results clearly show that foaming increases with an increase in each of the two types of particles but to a different degree. The presence of biphilic particles causes a significantly higher degree of foaminess than the hydrophilic colloidal particles.