The effect of sodium 5-sulfoisophthalic acid (NaSIPA) on the solid-state polymerization of hexamethylenediammonium adipate was studied. In particular, different polyamide salt grades, such as a model salt of hexamethylenediamine and NaSIPA and a polyamide 6,6 salt containing NaSIPA, were prepared through alternative procedures based on the solution-precipitation technique. Furthermore, selected salt grades were solid-state-polymerized in a thermogravimetric analysis chamber under static and flowing nitrogen. Critical reaction parameters, such as the reaction temperature, surrounding gas, and presence of NaSIPA, were investigated to determine the rate-controlling mechanism of the process. More specifically, NaSIPA significantly influenced solid-state polyamidation by reducing the reaction rate and changing the prevailing mechanism. (c) 2007 Wiley Periodicals, Inc.