The effect of molecular structure on the gamma relaxation dynamics has been studied in a set of aromatic poly(isophthalamide)s. This polymer family differ in the bridge group between phenylene rings [hexafluoroisopropylidene (C(CF3)(2)) or ether] and also in the presence of t-butyl groups (C(CH3)(3)) as pendant substituent on the five position of isophthalic ring. The results obtained from wide angle x-ray scattering in the glassy state indicated that both (C(CF3)(2)) and (C(CH3)(3)) groups favor the separation between chains, which is reflected on different interchain average distances. Dielectric experiments showed that both bulky groups favor the mobility in the glassy state. Molecular modelling methods were used to know the kind of molecular motions associated to the dielectric, relaxation observed below the glass transition temperature. (C) 2004 American Institute of Physics.