Molecular dynamics (MD) simulation with the TRIPOS 5.2 force field was carried out on a phenylene polymer, poly(aryl ether ether ketone), in its amorphous state at two temperatures. Rotations of single phenylene rings and large segments involving five consecutive phenylene rings and four bridging atoms were investigated. The activation energies for the restricted cooperative rotations of both types of rotational units were estimated around 1.5 kcal/mol. For the restricted phenylene ring rotations, the ether-ether rings were found to be more mobile than the ether-ketone rings. On the average, the restricted rotations of large segments are less mobile than the single phenylene rings. The local environment of these large segments was found to be very similar to that in the amorphous poly(phenylene oxide).