The bromination of poly(1-trimethylsilyl-1-propyne) (PMSP) was carried out by immersing a PMSP membrane in bromine water at 25-degrees-C. The bromine mainly reacted at the carbon-carbon double bonds in the backbone chain, and carbon-carbon single bonds were produced, which was determined from the infrared (IR) and ultraviolet (UV)-visible analyses. The glass transition temperature of the PMSP is above 350-degrees-C, but a new endothermic peak appeared between 50 and 80-degrees-C in the differential scanning calorimetry (DSC) curves of all brominated PMSPs. The permeability for 12 gases in the PMSP membrane and its brominated membranes was investigated between 30 and 90-degrees-C below 1 atm. With increasing bromine content in the membrane, the permeability coefficient for all gases decreased together with the diffusion coefficient, and the ideal separation factor for the industrially important gas pairs increased at 30-degrees-C. A distinct change in slopes at near the endothermic temperature determined by the DSC analysis was observed in Arrhenius plots of the permeability coefficients in all brominated PMSP membranes.