Data on the effect of internals, esp. motionless mixers, on the behavior of fluidized-bed reactors are provided. The influences of internals on the expansion characteristics of fluidized beds is most pronounced for particles in the range of Archimedes nos. <50. They cause an increase of the operating range in terms of surface velocity by a factor of 5. Internals also prevent channeling and the creation of large recirculation patterns. [on SciFinder (R)]