The pure rotational spectrum of AsD 2 in the 2 B 1 ground electronic state was measured in the frequency region of 268 -398 GHz by microwave spectroscopy. The AsD 2 radical was generated in a free-space cell by dc-glow discharge of a gaseous mixture of D 2 and He over arsenic powder. Fifty fine and hyperfine components of nine rotational transitions were measured and analyzed by least-squares methods determining the detailed molecular constants of AsD 2 : the rotational constants, the centrifugal distortion constants, the spin-rotation coupling constant with the centrifugal distortion terms, and the hyperfine coupling constants associated with the arsenic and deuterium nuclei. The harmonic force field of AsH 2 was determined from the centrifugal distortion constants, the equilibrium vibrational frequency of 2 mode of AsH 2 , and the inertial defects of AsH 2 and AsD 2 . The harmonic force field was used to calculate vibrational corrections to the rotational constants of both species and thus the zero-point average structures for both species were derived; AsH 2 : r z (AsH) ϭ 1.53436(15) Å, z (HAsH) ϭ 90.695(20)°, and AsD 2 : r z (AsD) ϭ 1.52902(13) Å, z (DAsD) ϭ 90.722(17)°, where the errors in parentheses are due to residual inertial defects of the corrected moments of inertia. From the difference between the r z structures for AsH 2 and AsD 2 the r e structure of AsH 2 is estimated to be r e (AsH) ϭ 1.5158(6) Å, e (HAsH) ϭ 90.79(8)°.