For the most part, ternary polymer blends demonstrate complete wetting behavior. Conceptually, this is the state where one of the components will always tend to completely separate the other two and from a thermodynamic viewpoint is described as the case where two of the three possible binary spreading coefficients are negative and the other is positive, as defined by Harkins spreading theory. This work examines the complete range of morphological states possible for such a system over the entire ternary composition diagram as prepared by melt mixing. A ternary polymer blend comprised of high-density polyethylene (HDPE), polystyrene (PS), and poly(methyl methacrylate) (PMMA) is selected as a model system demonstrating complete wetting and four sub-categories of morphologies can be identified including: a) matrix/core shell dispersed phase; h) tri-continuous; c) matrix/two separate dispersed phases, and d) bi-continuous/dispersed phase morphologies. Electron microscopy as well as a technique based on the combination of focused ion beam irradiation and atomic force microscopy are used to clearly illustrate and identify the various phases. Solvent extraction/gravimetry is used to examine the extent of continuity of the systems so as to effectively identify regions of high continuity. Triangular compositional diagrams are used to distinguish these various morphological regions and the results are interpreted in light of the interfacial tension of the various binary combinations and their subsequent spreading coefficients. The effect of the molecular weight and of viscosity ratio on the phase size of the various structures is also considered. (C) 2010 Elsevier Ltd. All rights reserved.