The present investigation deals with the fine structural characteristics of heated and alkali-treated sisal fiber using small-angle X-ray scattering (SAXS) technique for nonideal two-phase systems. The SAXS intensities of heated and alkali-treated sisal fibers deviate from Porod's law, indicating that the samples belong to nonideal two-phase systems characterized by continuous variation of electron density at the phase boundary. The macromolecular parameters such as the average periodicity transverse to layers, specific inner surface, volume fractions of void and matter phases, and so on of the samples were evaluated by using one- and three-dimensional correlation functions, which reveal all the information concealed in the slit smeared SAXS patterns. As all the samples under our purview of study have two-phase structure with diffuse phase boundaries, the average thickness of the transition region have been determined. This analysis throws some light on the structural changes that occurred in the dewaxed sisal fiber treated by NaOH solutions and subject to heat treatments. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012