The small-angle X-ray scattering (SAXS) method has been applied to evaluate various macromolecular parameters such as the specific inner surface, transversal lengths, the length of coherence, the range of inhomogeneity and the percentage of air present in sisal (Agave sisalana) fibre after treating the fibre according to the technically important process of mercerisation. The samples are found to behave as a densely packed two-phase system belonging to a general micelle system. The theories of O. Kratky (1966) and G. Porod (1960) have been utilised to evaluate the macromolecular parameters. The small-angle Kratky camera is used for photographic registration of the scattering intensities. Monochromatised CuKalpha radiation is obtained by reflecting the copper radiation from a Johansson-Guinier bent-crystal monochromator. Comparison of the scattering curves showed that these treatments dissociate the scattering inhomogeneities without improving the mechanical properties and the appearance of the fibre.