This paper presents a mechanism to explain the attachment of solid particles to air bubbles in surfactant-free aqueous solutions where both solids and air bubbles have the same sign of zeta potential via investigating the mechanical properties of micro air bubbles and the adsorption of hydroxide on air bubble surfaces. Particle-bubble attachment was measured in a Hallimond tube. The results indicate that purified quartz particles attached to air bubbles in surfactant-free deionised water, and the attachment increased with the pH of the aqueous solutions. The mechanical properties of micro air bubbles in aqueous solutions were measured using a novel micromanipulation technique. It was found that the micro air bubbles were pseudo-elastic and spherical in the solutions. The rigidity of the air bubbles decreased with increasing pH of the solutions. When a moving particle with a certain kinetic energy collided with an air bubble in a surfactant-free aqueous solution, the deformation of the air bubble varied with pH of the solution. In an alkaline solution, the micro air bubble was much softer and the deformation was larger than that in an acidic solution. The larger deformation of the softer air bubble resulted in a large contact area between the solid particle and the air bubble, therefore increasing the attachment, and reducing the rebound. The attachment of purified quartz particles to air bubbles in surfactant-free aqueous solutions was possibly due to hydrogen bond formation. The OH- ions on air bubble surfaces formed hydrogen bonds with silicon and oxygen atoms in dropSi-O-Sidrop or with the adsorbed OH group on quartz surfaces. (C) 2004 Elsevier Ltd. All rights reserved.