Fluid bed reactors for decomposing metal carbonyls and comprising a vertically disposed, elongated vessel for confining a bed of fluidized particulate material are provided with heating coils, that can be heat exchange coils through which heating fluid is passed or electrical resistance heating coils, in the lower part of the reactor and fluidized bed so that the lower portion of the fluidized bed functions as a heating region while the upper portion functions as a decomposing zone. The lower and upper portions of the fluidized bed are volumetrically proportioned to provide substantially equal gas velocities to promote conveyance of heated particulate material from the heating region to the decomposing region. A process for decomposing metal carbonyls is also disclosed in which separate heating and decomposing regions in a fluidized bed are established and a metal carbonyl is introduced into the decomposing region via a water-cooled lance to be decomposed on heated particulate material.