The gas-phase structure of perchloric acid (HOClO3) was determined by joint analysis of the electron diffraction intensities and rotational constants. The structures of the halogen perchlorates ClOClO3 and FOClO3 were derived from electron diffraction data alone. The geometric parameters of the ClO3 group (r(2) distances and angle(alpha) angles with 2 sigma (HOClO3) or 3 sigma uncertainties) are nearly constant in this XOClO(3) series : Cl=O = 1.407(1), 1.408(2), and 1.406(3) Angstrom and O=Cl=O = 114.9(2), 115.4(8), and 115.2(9)degrees for X = H, Cl, and F, respectively. The Cl-O single bond, however, lengthens strongly with increasing electronegativity of X : Cl-O = 1.641(2), 1.679(10), and 1.702(5) Angstrom for X = H, Cl, and F. Ab initio calculations at various standard levels (HF/3-21G(*), HF/6-31G(*(*)), and MP2/6-31G(*(*))) reproduce the Cl=O double-bond lengths and the bond angles reasonably well. The predicted Cl-O single-bond distance and the O-F bond length in FOClO3, however, depend strongly on the calculational method. The calculated Cl-O bonds are about 0.04 Angstrom too short at the HF level and 0.05-0.12 Angstrom too long at the MP2 level. The experimental O-F bond distance (1.427(15) BL) is Slightly Shorter than the HF/3-21G(*) (1.443 Angstrom) and MP2/6-31G(*) (1.442 Angstrom) results, whereas the HF/6-31G(*) method(1.359 Angstrom) predicts this bond length to be much too short.