Mixed-matrix membranes (MMM) consisting of poly(1-trimethylsilyl-1-propyne) (PTMSP) as continuous matrix and small-pore LTA-framework zeolites with Si/Al ratios from 1 (commercial zeolite A) to ¥ (ITQ-29) as dispersed phase were prepared by solution casting. The thermal stability of the MMM is as high as that of glassy PTMSP polymer, whose high permeability is maintained even at increasing temperature. The effect of the Si/Al ratio in the zeolite fillers on the membrane performance is observed by the increasing CO2/N2 permselectivity of low-Si/Al ratio zeolite A-based membranes, in comparison with pure silica ITQ-29. The performance of the LTA-type zeolite-PTMSP MMM was adjusted to the modified Maxwell model by estimating the chain immobilization factor and the interphase thickness as a function of temperature, Si/Al ratio, and zeolite loading. ; Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) under project CTQ2012-31229 at the Universidad de Cantabria is gratefully acknowledged. A.F.B. and C.C.C. also thank the MINECO for the Early Stage Researcher (BES2013-064266) and “Ramón y Cajal” (RYC2011-0855) grants, respectively. M.P. and S.V. also gratefully acknowledge the financial support from the Spanish Government (MAT2012-38567-C02-01, Consolider Ingenio 2010-Multicat CSD-2009-00050 and Severo Ochoa SEV-2012-0267).