AbstractAtomistic packing models have been built for PIM-1, a polymer belonging to a recently developed new type of “polymers of intrinsic microporosity” (PIMs). The molecular structure of PIM-1 contains sites of contortion (spiro-centres) within a rigid ladder polymer structure. Nitrogen sorption experiments have shown that the polymer, which traps in the glassy state a large amount of interconnected free volume, behaves in many respects like a microporous material. The models obtained by force field-based molecular mechanics and molecular dynamic methods allow a prediction of permeation properties of small gasses, the calculation of N2-sorption isotherms at 77 K, and a determination of free volume distributions. The Gusev–Suter transition state theory (TST) was used to calculate gas solubility and diffusion coefficients for He, H2, Ar, O2, N2, CH4, and Xe. Good agreement with experimental data (factors 1–4) was found. The calculated nitrogen adsorption isotherm at 77 K shows properties typical for sorption on microporous materials. The obtained microporous volume is in good agreement with the respective experimental value. The size distribution function of free volume elements derived from the models agrees well in its range of pore width of 5–15 Å with results obtained from experimental sorption data and analyzed with the Horvath-Kawazoe method. Dihedral angle distributions in the backbone reveal a certain, unexpected flexibility of plane segments between the spiro-centres which act as connecting links.