The amine end groups of polyamide 6 (PA-6) granules were blocked with succinic anhydride (SA) in supercrit. or subcrit. fluids. The fluids were mixts. of either propane or CO2 with 10 mol% 1,4-dioxane. Temps. of 50-140 DegC and pressures of 70-80 bars were applied. The reactions were followed in time. The modified polyamide samples were analyzed by amine end group titrns., by NMR and FTIR spectroscopy, by soln. and melt viscosity measurements and by SEC/DV. Sorption measurements were performed to det. the uptake by the PA-6 granules of either CO2, CO2/1,4-dioxane satd. with SA, or CO2 satd. with SA. End group concns., relative viscosities and mol. wt. 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 temp. However, at 140 DegC the granules discolored and for PA-6 modified in supercrit. CO2/1,4-dioxane, a decreased mol. wt. was found. Optimum results were obtained at 100 DegC in propane/1,4-dioxane: a decrease in amine end groups provided a better melt stability without discoloration or a change in mol. wt.