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The application of supercritical fluids (SCF) to the technological processes of fibre treatment is becoming increasingly important. Therefore we investigated highly oriented semicrystalline unmodified poly(ethylene terephthalate) fibres by means of SAXS in order to study the influence of supercritical CO2 (critical data: Tc = 304.2 K, Pc --7.38 MPa), an environmentfriendly dyeing media, on the lamellar structure and on the content, size and shape of microvoids in the fibres.Blind dyeing was performed in an autoclave after two different methods: the isothermal-isobaric method and the temperature-pressure step method.To elucidate the effects of the treatment conditions on the microvoid system and the lamellar colloid structure, scattering curves of untreated and treated fibres were determined in equatorial and meridional directions, and small-angle x-ray photographs were taken as well.Several parameters pertaining to the void system and the long period of the fibres were derived therefrom. The results suggest that the microvoid system of the fibres is more influenced by the SCF than the lamellar fibre structure itself. As the treatment conditions in SCF were adapted to the dyeing conditions, they are probably too low to cause considerable changes in the two-phase fibrillar structure. Within the treatment levels used only the highest temperature changes the long period significantly, but there is no significant difference in the crystallinity according to the treatment conditions.The content of the free volume in the untreated PET fibres is very low, as expected. Due to the physical treatment at the SCF conditions, some modifications of the microvoid system were observed.