A simple method for purifying crude hexanitrohexaazaisowurtzitane (HNIW or CL20) to analytical purity is described. This involves filtering a solution of HNIW in heptane/ethyl acetate through a column of Darco activated carbon type G-60 (similar to 20 g carbon/g HNIW). The method works extremely well for HNIW derived from 4,10-diformyl-2,6,8,12-tetraacetylhexaazaisowurtzitane (final purity >99.95%, similar to 90% recovery), and reasonably well for HNIW derived from 4,10-dibenzyl-2,6,8,12-tetraacetylhexaazaisowurtzitane.