A series of blends of high molecular weight polyamide 11 (PA11) and three different polar molecules [i.e., N-butyl benzene sulfonamide (BBSA), delta-valerolactam, and omega-laurolactam) have been studied by differential scanning calorimetry (DSC), Fourier transform infrared microscopy (FTIR), differential mechanical thermal analysis (DMTA), and wide- and small-angle X-ray scattering (WAXS and SAXS) experiments. FTIR analysis shows that the concentration of free NH groups in bulk or plasticized PA11 is lower than 15% up to 260 degrees C and less than 1% at room temperature. DMTA data show a beta relaxation for dry PA11 but not for dry PA6 and PA12. On the other hand, when omega-laurolactam, which allows trans conformation of amide groups, is added to PA11 the intensity of the alpha relaxation increases and a strong antiplasticizing effect is observed. This effect is associated with a decrease of the PA11 free volume by increasing the chains packing. On the contrary, when delta-valerolactam, which allows cis amide groups conformations, and BBSA are blended with PA11, the alpha relaxation temperature and beta peak intensity decrease as a function of added molecules concentration. This is associated with a plasticizing effect. It is suggested to attribute the bulk beta relaxation to segmental motions involving H-bonded CONH in a cis conformation in amorphous domains of the PA11. In turn, the alpha relaxation is related to segmental motions involving CONH groups in a trans conformation. Therefore, antiplasticizing and plasticizing effects depend upon the ability of the additive molecule to change the initial conformational structure of polyamide (i.e., the ratio cis over trans H-bonding of the amide group conformations). Overall, a point of interest to note seems to be the difference between the trans and cis conformations of the amide groups in term of bonding with each other in a PA11 chain and additives such as BBSA. In addition, the limited influence of BBSA on the crystalline microstructure of PA11 is explained by the fact that 85% of the PA11 amorphous phase is intraspherulitic and that a great part of the plasticizer is located in these domains.