The characteristics of bubbles and particles in dissolved air flotation (DAF) will govern the collision efficiency between bubbles and particles and thereafter the removal efficiency of a suspension. In this paper, the collision efficiency factor for bubbles and particles (αbp) is calculated from a trajectory analysis by a method similar to that used for the calculation of the collision efficiency factor in differential sedimentation (αds) (Han & Lawler 1991). The effects of the contributing parameters in the bubble-particle-solution system are investigated. The most important parameters in the DAF system are found to be the surface characteristics (zeta potential) of both bubbles and particles. The next most important parameters are the sizes of the bubbles and the particles. The density of the particles has a positive or negative effect on αbp depending on the sizes of particles and bubbles. The ionic strength of the solution affects the αbp slightly. The theoretical results can explain some of the current practices in the design and operation of the DAF process.