Monte Carlo (MC) simulations have been used to study the crystal structure of isotactic poly(4-methyl-1-pentene). Four different tetragonal packing models, each one containing two right-handed and two left-handed 7/2 helices, have been considered in an investigation of the up-and-down chain statistical disorder proposed on the basis of X-ray data. Simulations have been performed with the isotropic united-atom parameterization of the AMBER force field. The influence on the more stable packing models of the force-field parameterization has been investigated with respect to the anisotropic united-atom and all-atom models. Results reveal that packing consisting of two upward and two downward helices arranged at random is more stable than packing with three or four helices with the same sense. Furthermore, the fiber period length for the 7/2 helix is predicted to be 0.56 Angstrom larger than that experimentally determined. The microstructures generated from MC simulations have been employed to study the solubility of gas molecules (He, H-2, Ar, O-2, CH4, and CO2) with Widom's test-particle insertion method. Special attention has been paid to the solubility of CH4 and CO2 because experimental data are available for these penetrants, the latter being described by both spherical and explicit models. The results are in good agreement with experimental measures only when a suitable model is used for the penetrant. The solubility of gas molecules in crystalline poly(4-methyl-1-pentene) has been correlated with that measured for the crystal phases of other helical polymers.