Particle carry-over from the surface of a fluidized bed into the freeboard depends onthe mechanism of bubble eruption. Particles may be ejected into the freeboard from the roofs or wakes of bursting bubbles, depending on bed geometry, fluidizing velocity U and particle type. There are believed to be two alternative ejection mechanisms, as follows: (1) For group B particles with U/U mf < 10–15, ejection is from the roof of each bursting bubble. As a bubble breaks through the surface, its roof attenuates; when the thickness of the roof equals the mean diameter of the particles in the bed, all roof particles are ejected into the freeboard. (2) For group A particles, and for group B particles with U/U mf ⪢ 10–15, ejection into the freeboard is predominantly from bubble wakes, because coalescence is simultaneous with eruption. Rates of surface carry-over and particle ejection velocities are much greater than for mechanism (1). There is some discussion of the effect of segregation on surface carry-over of fine particles.