A new set of topological parameters describing connectivity of a phase-separated bicontinuous structure of a polymer mixture in the late stage of spinodal decomposition has been directly measured by use of laser scanning confocal microscopy. A 3D thinning algorithm was used to transform one of the phase-separated domains into a skeletal network that locates at about the center of the domain. Number of channels intersecting at a junction, N-j, and distance between the adjacent junctions, D-j, were obtained from the resulting skeletal network. It was found that the phase-separated domain of the polymer mixture possesses, for the most part, 3 branches at each junction. The Euler characteristic, describing topology of the network, was found to be between similar to-2 to -4, topologically equivalent to a sphere with similar to2 to 3 handles. Time-evolution of the histograms of the coordination number, H(N-j), and distance, H(D-j), clearly demonstrates that the bicontinuous structure grows with self-similarity in topological sense.(C) 2003 American Institute of Physics.