The structure of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is distinct from other glassy polymers because of the presence of bulky side groups which obstruct the packing of polymer chains and allow extraordinarily high permeability of sorbed molecules. Molecular transport models traditionally applied to standard glassy polymers reach some limitations in the description of diffusion in PTMSP because of differences in the mechanism of sorption and diffusion. Analysis of sorption equilibria and permeability data measured for PTMSP is undertaken in an effort to elucidate the mechanism of mass transfer. Sorption equilibrium indicates that a multilayer adsorption mechanism can account for the isotherm data, and analysis of dynamics indicates that a multilayer surface diffusion model successfully characterizes the permeability measurement. A favorable comparison of values for the characterizing variables between PTMSP and other microporous materials generates further support for the assertion that this high free-volume glassy polymer can also be viewed as a microporous material.