The microwave spectrum of the transient phosphine oxide, H3PO, was detected in the gas phase for the first time using a source-modulated spectrometer. The H3PO molecule was generated in a free space cell by a de glow discharge of a mixture of PH3, CO2, and H-2 gases and the corresponding spectral lines for each rotational transition of H3PO formed a pattern clearly indicative of a symmetric top molecule. Isotopomers of H3PO, i.e., (H3PO)-O-18 and D3PO were also produced; (H3PO)-O-18 in the same fashion as for H3PO, and D3PO, in a mixture of D-2 and CO2 gases passing over red phosphorus grains. In total, 54 spectral lines of N3PO, 55 lines of (H3PO)-O-18, and 61 lines of D3PO were measured for the fine structure of rotational transitions, J=2-1 to 12-11. The rotational constant, B-0, and centrifugal constants, D-J, D-JK, and H-KJ were ascertained by a least squares analysis of the measured frequencies for each of the species. Using the rotational constants of the three isotopic species, the following r(0) structure was determined: r(0) (PO)=1.4763 Angstrom, r(0)(PH) = 1.4406 Angstrom, and angle HPO=114.26 degrees. The value of each structural parameter deviates significantly from the most recent results predicted by ab initio calculations.