The amine end groups of polyamide 6 (PA-6) granules were blocked with succinic anhydride (SA) in supercritical or subcritical fluids. The fluids were mixtures of either propane or CO2 with 10 mol% 1,4-dioxane. Temperatures of 50-140degreesC and pressures of 70-80 bars were applied. The reactions were followed in time. The modified polyamide samples were analysed by amine end group titrations, by NMR and FTIR spectroscopy, by solution and melt viscosity measurements and by SEC/DV. Sorption measurements were performed to determine the uptake by the PA-6 granules of either CO2, CO2/1,4-dioxane saturated with SA, or CO2 saturated with SA. End group concentrations, relative viscosities and molecular weight distributions of modified samples were compared with corresponding values of unmodified PA-6. It was found that better blocking of the end groups was obtained with increasing reaction temperature. However, at 140degreesC the granules discoloured and for PA-6 modified in supercritical CO2/1,4-dioxane, a decreased molecular weight was found. Optimum results were obtained at 100degreesC in propane/1,4-dioxane: a decrease in amine end groups provided a better melt stability without discolouration or a change in molecular weight. (C) 2003 Elsevier B.V. All rights reserved.