We report a novel composite membrane made from the high free volume polymer poly( trimethylsilylpropyne) and the small organic filler trimethylsilylglucose (TMSG). The permeabilities, diffusivities, and solubilities of six gases (He, H-2, CO2, O-2, N-2, CH4) were determined in these PTMSP/TMSG composites with a series of TMSG loads using the time-lag method. Increasing TMSG content in PTMSP resulted in substantial reduction of all gas permeabilities. The observed decrease in permeability was much larger than predicted by the Maxwell model for the incorporation of impermeable fillers. In addition, the permeability loss varied significantly from gas to gas, leading to increased selectivities for some gas pairs. For example, nitrogen permeability (9.6 x 10(-10) cm(3) (STP) cm/(cm(2) s cmHg)) in PTMSP containing 56.8 vol % TMSG decreased by more than 600-fold compared to that of unfilled PTMSP (5490 x 10(-10) cm(3) (STP) cm/(cm(2) s cmHg)). Simultaneously, the O-2/N-2 selectivity increased from 1.5 up to 3.4. The varying permeability behavior in PTMSP/TMSG composites is in good agreement with the diffusivity change. In addition, a parallel reduction in solubility for all tested gases was observed. In these composites, the natural logarithms of the diffusivities and solubilities are well linearly related to the square of penetrants diameter and their condensability, respectively. It was observed that the activation energy of permeation increased with TMSG content. From the analysis of the temperature dependence of the gas permeability, we conclude that the gas transport in pure PTMSP and PTMSP/TMSG composites follows different mechanisms. Our results indicate that the PTMSP/TMSG composite membranes offer a readily accessible means to physically modify the FFV in PTMSP polymer and to achieve the desired gas permeability and permselectivity compared to the pure polyacetylene polymers.