Particle mixing and segregation have been studied in a small-scale fluidized-bed reactor (FBR) under pressure. The solids mixing is relatively faster than the residence time of catalyst particles in the case of a polymerization process, but smaller particles accumulate in the upper zone. Semibatch propylene polymerization experiments showed that the vertical temperature gradients are caused mainly by catalyst segregation. At low gas velocities, segregation and mixing can differ under reacting conditions compared to nonreacting conditions due to particle-particle interactions. Catalyst concentration gradients caused by incomplete mixing are strengthened remarkably by the exothermic reaction even at low polymerization rates. These observations do not represent an industrial situation. The FBR has therefore been equipped with a draft tube and cone to control vertical solids mixing. The internal solids circulation rate is a nonlinear function of the gas velocity. Strongly reduced segregation, elutriation, and entrainment observed were compared to experiments without a draft tube. Temperature profiles observed during polymerization can be controlled by the solids circulation rate. Hydrogen injections led to an instantaneously increased polymerization rate, probably due to the reactivation of dormant sites. Irreversible deactivation rates of dormant and active sites seem to be the same. Moreover, hydrogen appeared to be very effective for widening the molecular-weight distribution. A compartment model developed describes the temperature profile in the reactor and related molecular-weight distribution of the polymer.