The pore-size distribution of pyrocarbon (PyC) coatings was measured by small-angle x-ray scattering. The method proved to be a very sensitive means of characterizing the microstructure of PyC’s. Variations in deposition conditions — such as temperature, hydrocarbon gas, and gas flux—lead to distinct variations in pore-size distribution. Different pore-size intervals are related to such material properties as density and fracture stress as well as to the irradiation stability and gas permeability of low-temperature isotropic (LTI) coatings. A parametric property, the relative amount of fiber component, is derived from the pore-size distribution and correlated with the microstructural components of LTI coatings as well as with the irradiation performance.