Copolyamides of PA 66/6 lithium 5-sulfoisophthalic acid (LiSIPA) containing up to 40 mol % of LiSIPA were prepared in a 1L-pilot reactor operating at high pressures and high temperatures. Interestingly, the presence of lithium sulfonate moieties highly impacted the glass transition temperature of the polyamide. The T(g) increased from 59 degrees C for PA 66 to 155 degrees C for a copolymer containing about 40 mol % of LiSIPA. 1,3-Dihexylbenzenedicarboxamide and lithium p-toluenesulfonate were synthesized as model compounds to investigate the interaction of lithium sulfonate moieties and amide functions. Infrared spectroscopy using ATR technology performed on mixture of both compounds showed that the carbonyl group of amide functions interacts with the lithium cation of lithium sulfonate moieties. Similar S-O and C-O adsorption bands were observed in copolyamides PA 66/6LiSIPA and in mixture of model compounds, which strongly suggest the formation in the copolyamides of physical cross-linking points centered on lithium cations coordinated by carbonyl groups of amide functions. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5057-5062, 2011