Investigations of the size distribution of nanopores were carried out for alumina coatings deposited by the impulse plasma method. The single-phase (metastable and stable correspondingly) alumina coatings were deposited under different conditions of impulse plasma processes. The investigation of nanopore distribution was carried out using small-angle X-ray scattering. Despite the different phase composition of the coatings obtained, the most probable value of nanopores for both alumina coating materials were practically the same and equal to 5 nm. It appears that a coating porosity of the order of nanometres is characteristic for all coatings deposited by the impulse plasma method, because previously similar dimensions of nanopores were found for diamond, TiN and BN. It seems that during the impulse plasma deposition the coating grows on the substrate surface by condensation of ultra-small particles nucleated in the impulse plasma.