A molecular dynamics (MD) simulation of a simple phenylene polymer, poly(phenylene oxide) (PPO), was carried out with the TRIPOS 5.2 force field. Two types of molecular motion were examined : rotation of individual phenylene rings and torsion of a larger segment (i.e., a four-oxygen segment (FOS)) involving four ether oxygen atoms and three phenylene rings. Model compounds, diphenyl ether and 1,4-bis(4-phenoxyphenoxy)benzene, were used to help the analysis of the results. Based on the trajectory analysis, "in-phase" cooperative rotations of neighboring phenylene rings and "out-of-phase’ cooperative rotations of consecutive FOS’s superimposed on random segmental wiggles, were observed. Packing effects were found to be important for the larger FOS rotations but not for the rotation of individual rings. The diffusion coefficient for the torsion of an FOS is about half of that for the rotation of an individual phenylene ring but remains within the same order of magnitude.