The far-infrared spectra of cyclopentanone have been reexamined. In addition to the ring-bending series previously observed in the 80-100 cm(-1) region, ring-twisting bands (200-240 cm(-1)), bend-twist sum bands (305-335 cm(-1)), and bend-twist difference bands (120-145 cm(-1)) were also detected for the first time. From these data a two-dimensional potential energy surface with a barrier to planarity of 1408 cm(-1) and a barrier to pseudorotation of 1358 cm(-1) was determined. The surface has energy minima at twist angles of +/-29 degrees. This surface yields calculated frequencies in good agreement for not only the undeuterated cyclopentanone, but also for four isotopomers. We also utilized our previously reported fluorescence excitation spectra to determine the corresponding potential energy surface for the S-1(n,pi*) electronic excited state. The barrier to planarity in the S-1 state is 1445 cm(-1) while the barrier to pseudorotation has been reduced substantially to 596 cm(-1) The twist angles are +/-30 degrees.