The permeability of poly(dimethylsiloxane) (PDMS) and poly(1-trimethylsilyl-1-propyne) (PTMSP) to a simulated syngas feed containing H-2, CO, CO2, and H2S was determined as a function of temperature up to 240 degreesC (464 degreesF). The permeation properties of rarely studied CO and H2S were found to be consistent with their molecular properties (i.e. critical temperature) in both rubbery PDMS and high free volume, glassy PTMSP At room temperature, PDMS and PTMSP are more permeable to the more condensable gases CO2 and H2S than to H-2. However, both polymers become hydrogen selective at elevated temperatures. Activation energies of permeation are highest for H-2 in both polymers and decrease regularly with increasing gas condensability. PTMSP exhibits evidence of accelerated physical aging at high temperature.