The (A) over tilde (1)A"-(X) over tilde (1)A(') electronic spectra of jet-cooled HPO and DPO have been studied using the techniques of pulsed discharge jet, laser-induced fluorescence, and wavelength resolved emission spectroscopy. All of the vibrational frequencies in the ground and excited states have been obtained for both isotopomers and vibrational force fields have been determined for both states. Rotational analysis of the high-resolution 0(0)(0) band spectrum of DPO has yielded the first rotational constants of the deuterated species. By combining the rotational constants of DPO with literature values for the rotational constants of HPO, we have derived reliable structures of HPO in the combining states with estimated equilibrium values of r"(PH)=1.4578(6) Angstrom, r"(PO)=1.4801(1) Angstrom, theta"=104.62(7)degrees, r(')(PH)=1.4671(26) Angstrom, r(')(PO)=1.5579(6) Angstrom, and theta(')=97.4(4)degrees. The decrease in the bond angle on n-pi* electronic excitation is contrary to predictions based on Walsh diagrams. A quantitative ab initio study shows that the variation of the orbital energies with bond angle differs in the ground and excited states of HPO, and these differences account for the anomalous change in bond angle on electronic excitation.