This work concerns the preparation and characterization of ITQ-29 zeolite crystals with high Si/Ge ratio (100–∞) and different particle size for obtaining mixed matrix membranes. The Si/Ge molar ratio and seeding content of the synthesis gel appeared to have an effect on the final crystal size, and particles of 2.5 μm were obtained for pure silica composition with good crystallinity. These were introduced, at 4, 8 and 12 wt% loadings, into a commercial polysulfone matrix to prepare mixed matrix membranes that showed promising results in the separation of H2/CH4 mixtures (highest H2 permeability 21.9 Barrer and a separation factor of 118 for the 4 wt% ITQ-29/polysulfone membrane). The thermal treatment of the membranes and the type of solvent were also optimized to provide good interaction between the zeolite and the polymer, limiting aggregation of the particles in the matrix, and removing all residual solvent that hinders gas permeation performance. ; Spanish Ministry for Innovation and Science (MAT2007-06128, MAT2009-1458-C02-01 and CIT-420000-2009-32) ; Ministry for the “Juan de la Cierva” program and the Government of Aragón for the Mobility Grant FMI016/08 ; Casado Coterillo, C.; Soto, J.; Jimaré Benito, MT.; Valencia Valencia, S.; Corma Canós, A.; Téllez, C.; Corona, J. (2012). Preparation and characterization of ITQ-29/polysulfone mixed-matrix membranes for gas separation: Effect of zeolite composition and crystal size. Chemical Engineering Science. 73(7):116-122. doi:10.1016/j.ces.2012.01.024. ; Senia ; 116 ; 122 ; 73 ; 7