Antimonic acid cannot be dehydrated by heating in air to give products of constant and reproducible weight without simultaneous reduction of some of the SbV to SbIII. Neither anhydrous Sb2O5 nor the hydroxy oxide Sb3O6OH postulated by Dihlström and Westgren can be obtained by this method. Two well-defined products of the dehydration/decomposition are Sb2O4.35(5) = Sb6O13, which forms between 650 and 850°, and β-Sb2O4. The latter, and not Sb2O3, results on heating Sb6O13 to 935°. Sb6O13 has a cubic structure of the defect pyrochlore type, a0 = 10.303(1) Å, x(O2) = 0.4304(14). Combined evidence from X-ray diffractometry, density determination, and Mössbauer spectroscopy leads to Sb3+Sb5+2O6O0.5 as the most probable structural formula.The Mössbauer parameters of β-Sb2O4 are closely similar to those reported for α-Sb2O4, but the isomer shifts (relative to InSb at 77°K) for SbV in antimonic acid and Sb6O13 are significantly larger than those in α- and β-Sb2O4.