The morphological similarity between a phase-separated polymer blend and a one-phase bicontinuous microemulsion is discussed in detail by comparing their respective light and X-ray scattering intensity distributions with structure factors computed according to the random wave model (RWM). In the RWM, a three-parameter spectral function introduced by Lee and Chen is used in conjunction with Berk’s theory and Cahn’s clipping scheme to calculate the Debye correlation function for the scattering intensity. The agreement between experiment and theory is excellent. The three parameters determined by fitting the scattering intensities are then used to calculate the average Gaussian curvature of the interfaces between the two isometric phases for the first time. The RWM can also be used to generate the 3D morphologies of both the bicontinuous phase-separated polymer blend and the one-phase microemulsion. There is a striking resemblance between the morphologies of these two unrelated systems, in spite of a difference in length scales characterizing the structures of a factor of about 250.