Pure rotational spectra of hypoiodous acid, HOI, and its deuterated species, DOI, were measured in the frequency range of 320-670 GHz. The molecule was efficiently produced by a reaction of atomic oxygen with iodoethane. Rotational constants and centrifugal distortion constants for the molecule were determined accurately. The vibrationally averaged structure for HOI was obtained by taking the isotopic difference of the OH bond length into consideration: r(z)(OH) = 0.967(8) Angstrom, r(z)(OI) = 1.9941(3) Angstrom(,) and theta(z)(HOI) = 103.89degrees, where the errors were estimated from the residual inertial defect. Equilibrium bond lengths for the OH and OI bonds were derived as 0.959(8) Angstrom and 1.9874(3) Angstrom, respectively, by assuming anharmonic constants of the corresponding diatomic molecules. Electric-quadrupole interaction constants and nuclear-spin-rotation coupling constants for the iodine nucleus were obtained. Nonaxial terms of the electric-quadrupole constant for HOI can be determined as well, which enabled us to derive the principal values of the coupling tensor. The values obtained were used to gauge the ionicity of the X-O bond in the HOX molecular system. The nuclear- spin-rotation coupling constant along the a inertial axis is found to be significantly smaller than others, which may be explained by a contribution from two low-lying singlet excited states. (C) 2004 American Institute of Physics.