Abstract In this paper, steady segregation and slugging experiments are presented which have been carried out with a binary large-particle system representing the ash—coal/sorbent mixture in a fluidized-bed coal combustor. The slugging beds are characterized by dimensionless slug parameters which are obtained from visual bed-height measurements. The influence of the superficial gas velocity and the bed diameter on the extent of particle segregation is investigated. It is shown that a critical bed diameter exists above which segregation is nil. A simple segregation model is introduced proceeding with a proposed mechanism of segregation in slugging gas fluidized beds, which is based upon the visual observed flow of particles. The segregating particle distribution is characterized by one ‘Peclet-like’ dimensionless number, which is the ratio between the net particle convective and segregating flows and the dispersive solids flow in the bed.