X-ray ptychographic tomography has been recently developed for quantitative imaging of
specimens on the nanometer scale. Here we present its application for the characterization
of carbon fibers by mapping in three dimensions the mass density of entire fibers with
diameters of several tens of micrometers with a resolution of about 100 nm. We character-
ized two fibers produced from two different precursors, revealing the spatial distribution of
porosity and highly graphitized regions within the fibers. We further discuss the potential
of ptychographic tomography as a new complementary technique for the characterization
of carbon materials.