Powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques were used to investigate variations of the structures of hexagonal birnessites as related to average oxidation states of manganese therein (Mn AOS). As the synthesized series of hexagonal birnessites from HB1 to HB6 decrease from 3.92 to 3.67 in Mn AOS, but increase from 2.838 Å to 2.848 Å in unit cell parameter b. The series also decrease from 12.0 nm to 7.0 nm in the coherent scattering domains (CSD) in the a -b layer plane, forming a stack of 10.6~13.4 layers of manganese oxide octahedron along axis c. With the decrease in Mn AOS, the content of octahedral voids declines from 18% to 8%. Mn AOS and vacancy content have a very significant positive linear relationship, and they both show negatively linear relationships with unit cell parameter b. Linear fitting of X-ray absorption near edge structure spectroscopy (XANES) shows that Mn in these samples is dominated by Mn⁴⁺ and the proportion of Mn^2+/3+ increases with the decreasing content of octahedral voids. Fitting analysis of Mn K -edge extended X-ray absorption fine structure spectroscopy (EXAFS) with a full multiple scattering model demonstrates that the samples, though different in Mn AOS, are similar in crystal structure and Mn local coordination environment. With the decrease in content of octahedral voids and particle size, apparent Mn site occupancy (f_occ) decrease from 0.74 for HB1 to 0.66 for HB6, and with the decrease in Mn AOS, the average bond-length of Mn-Mn(O) shells increase.