Structure analysis of unilamellar manganese oxide nanosheets obtained via exfoliation of layered manganese oxides was carried out utilizing synchrotron radiation (SR) X-ray in-plane diffraction and polarization-dependent total reflection fluorescence X- ray absorption fine structure (PTRF- XAFS) analyses. A combination of SR excitation and the total reflection of incoming X- rays provides signals strong enough for both analyses even from a monolayer of the MnO2 nanosheets having a concentration of 0.7 mu g cm(-2). In addition, the mean oxidation state of constituent manganese ions in the MnO2 sheets was estimated on the basis of XANES spectra, and bond valence sum calculations with the bond length obtained from the present EXAFS analyses. The obtained structural data revealed that the two-dimensional lattice of the MnO2 sheets underwent a slight elongation upon delamination. These changes correspond to approximately 1% expansion of sheet area and 1-2% expansion of thickness, which can be understood by reduction of the mean oxidation number of manganese ions in the sheet through the exfoliation process.