The effect of bulky side groups on the molecular mobility and chain packing of three polyimides derived from dianhydrides with m-terphenyl moieties and 2,4,6-trimethyl-m-phenylenediamine has been studied. Moreover, structure-properties relationships in terms of gas transport have been established. One of the polyimides has no side groups, while the other two have a t-butyl group in 5' (meta) position on the central ring of the m-terphenyl moiety and one of them has also a pivaloylimino group placed in the 2' (ortho) position. The changes in chain packing of the membranes were evidenced by wide angle X-ray diffraction (WAXS), whilst the molecular mobility was evaluated from the dielectric sub-ambient secondary relaxation, gamma relaxation. The WAXS pattern of the polyimide containing both t-butyl and pivaloylimino side groups evidenced a greater degree of packing regularity than in the other two polyimides. Moreover, the additional pivaloylimino group strongly restricted the local molecular mobility associated with the phenylene-imide ring flips. On the other hand, the gas permeability of the polyimides strongly increased with the introduction of side groups. This increase was mainly related to the increase in the gas diffusivity, which could not be explained by an increase in fractional free volume (FFV). The results presented in this work indicate that the difference in gas diffusivity of the substituted polyimides is consistent with a difference in size distribution of free volume elements. (C) 2013 Elsevier B.V. All rights reserved.