The millimeter-wave spectrum of P2O produced by reacting P-4 vapor with atomic or molecular oxygen directly inside the absorption cell has been observed in 13 vibrational states. Reliable identification of the various vibrationally excited states has been achieved with the help of high-level ab initio calculations resulting in accurate geometric data and an anharmonic force field. Careful analysis of the vibrational levels observed resulted in 13 sets of rotational and centrifugal distortion parameters, which have been used, in turn, to determine the equilibrium rotational, quartic centrifugal distortion, and rovibrational interaction constants of P2O. The 2 nu(1)-nu(3) Fermi resonance has been analyzed. (C) 1997 American Institute of Physics. [S0021-9606(97)01137-9].