The surface microstructures of specimens of pure alpha- and beta-lead dioxide (PbO2 ) have been resolved with scanning tunnel microscopy (STM). Observed crystallite sizes are compared with values deduced from x-ray diffraction (XRD), and the same data are compared with results from other characterization methods including scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurements, mercury porosimetry and electrochemical reduction. The large difference in surface area between alpha- and beta-PbO2 is explained by microcracks permeating the beta structure. The STM images show these microcracks clearly.