Solid state polymerization (SSP) starting from polyhexamethyleneadipamide (PA 6,6) precursors was studied in the temperature range of 160-200 degrees C in a solid bed reactor. Process parameters, such as temperature, time and inert gas flow rate were investigated, meanwhile emphasis was given on the effect of a dyeability modifier (sodium 5-sulfoisophthalic acid, NaSIPA) on the SSP rate, due to the importance of the pertinent sulfonated ionomers in the polyamide industry. Under the specific experimental conditions, the copolyamides with NaSIPA exhibited lower SSP rates than PA 6,6 homopolymer and their retarding behavior was found proportional to the comonomer content. A possible retardation mechanism related to the NaSIPA ionic species has been suggested and verified through modifying the kinetics so as to describe and predict the copolyamide SSP performance. (c) 2007 Elsevier Ltd. All rights reserved.