A series of dual-metal zeolitic imidazolate framework (ZIF) crystals with SOD and RHO topologies was synthesised by metal substitution from ZIF-108 (Zn(2-nitroimidazolate)2 , SOD topology) as the parent material. This was based on the concept that metal substitution of ZIF-108 requires a much lower activation energy than homogenous nucleation owing to the metastability of ZIF-108. In-depth investigations of the formation processes of the daughter ZIFs indicated that the transformation of ZIF-108 is a dissolution/heterogeneous nucleation process. Typical isostructural Co(2+) substitution mainly occurs at the outer surface of ZIF-108 and results in a core-shell structure. On the contrary, the Cu(2+) -substituted ZIF has a RHO topology with a homogeneous distribution of Cu(2+) ions in the structure. Substitution with Ni(2+) resulted in a remarkable enhancement in adsorption selectivity toward CO2 over N2 by a factor of up to 227. With Co(2+) -substituted nanoparticles as inorganic filler, a mixed matrix membrane based on polysulfone displayed greatly improved performance in the separation of H2 /CH4 , CO2 /N2 and CO2 /CH4 .