ZSM-5 zeolite membranes with Al, Fe, B, or Ge substituted for Si in the framework structure were prepared by in situ crystallization onto porous stainless steel supports. Their single-gas and mixture separation properties were compared to a silicalite-1 membrane. For all membranes, single-gas permeances at 473 K showed a decreasing trend as the kinetic diameter of the diusing molecule increased. All membranes separated n-C 4 H 10 /i-C 4 H 10 , n-C 4 H 10 /H 2 , and H 2 /i-C 4 H 10 mixtures, and the selectivities depended on temperature and the substituted cation, but no trend with acidity or hydrophobicity was observed. For most separations studied, the substituted membranes had higher selectivities than a silicalite-1 membrane, and the B-ZSM-5 membrane had the highest selectivities. The n-C 4 H 10 /i-C 4 H 10 separation was due to dierences in diusion rates and adsorption coverages, and the highest selectivity was 45. The n-C 4 H 10 /H 2 separation had a maximum value of 110 at 300 K, and separation was due to preferential adsorption of n-C 4 H 10 , which permeated faster than H 2 . For H 2 /i-C 4 H 10 mixtures, however, H 2 permeated faster, but i-C 4 H 10 adsorption reduced the H 2 permeation below the single-gas value so the mixture selectivity was below the ideal value.