Abstract The amine and carboxylic acid end groups of polyamide 6 (PA-6) granules were blocked with 1,2-epoxybutane in supercritical or subcritical fluids. The fluids were mixtures of either propane or CO 2 with 10 mol% 1,4-dioxane. Temperatures of 50–140 °C and pressures of 70–80 bars were applied. The reactions were followed in time. The modified polyamide samples were analysed by carboxylic acid end group titrations, by FTIR spectroscopy, by solution and melt viscosity measurements, by DSC and SEC/DV. End group concentrations, relative viscosities and molecular weight distributions (MWD) 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, after modification, at higher temperatures, a decrease in relative viscosity was found. SEC measurements showed that this decrease in relative viscosity not necessarily implies that chain scission had occurred, but that it must be the result of a different interaction of the modified polymer end groups with the solvent. Best results were obtained for PA-6 modified at 100 °C in propane/1,4-dioxane: an improved melt stability was obtained without discolouration of the PA-6 granules or a change in molecular weight, due to the blocking of the amine end groups but leaving the majority of the carboxylic acid end groups unblocked.