Porosity in several zirconia-based pressure compacted nanopowders was studied using the positron lifetime technique combined with the mass-density measurements. Two kinds of pores were identified: (i) the larger pores of ALMOST EQUAL TO 10 to 19 nm diameter arising likely from a formation of secondary particle aggregates, and (ii) the smaller ones (ALMOST EQUAL TO 1 nm) which are obviously of a more complex origin.