The width of domain boundaries is an important parameter for the characterization of microphase separation in polymers. This parameter is often obtained by measurements of small-angle X-ray scattering (SAXS) in the range of validity of Pored's law, according to which X-ray intensity scattered by an ideal two-phase system decreases with the scattering vector h as h(-4). Here h is defined as h = (4 pi sin theta)/lambda, where theta is half the scattering angle and h the wave-length of the X-ray. If the interface boundary is diffuse, i.e. the electron density does not change stepwise between separated phases but changes continuously in certain transition regions with a finite thickness, the intensity of scattering decreases more rapidly than predicted by Pored's law. In the present paper we have used the SAXS method in order io determine the supermolecular structure parameters, including the transition layer thickness, of polypropylene/polyamide-6 (PP/PA) blends. The transition layer thickness has been obtained by two methods elaborated by Koberstein and co-workers [4] and by Ruland [1], respectively. Both these methods assume that changes of the electron density in the transition region can be described by a Gaussian function with a standard deviation a. The parameter a have been determined graphically from the appropriate plots. Then, the thickness of the phase boundary, E, was estimated as root 12 sigma. The investigated PP/PA blends are multiphase systems and the problem of determination of the boundary width is more complicated because the meaning of Pored's law must be considered with caution. This problem is discussed based on a wide range of investigated samples prepared over various pressure and crystallization temperature conditions.