The recently expanding field of microstructured optical fibers relies on the controlled fabrication of sub-micron features in a fiber drawn in the viscous fluid state. Microstructured fibers have generated great interest owing to their potential in areas such as photonic bandgap guidance of light in low-index media; high-energy laser transmission; and unique control over waveguide non-linearities, dispersion and modal properties [1–6]. These fibers have been made from a single material with air holes [7, 8] and as multi-material ‘composite’ fibers where air is not a part of the microstructured region [6, 9]. While single-material microstructured fibers generally rely on the established technology base of fused silica, the use of less conventional materials may enable applications not possible using silica . Multi-material fibers may also present certain fabrication advantages due to their incompressible domains and simple cylindrical geometries. However, the use of more than one material raises questions about which types of materials can be combined in the drawing of a microstructured fiber. This problem can be approached by analyzing the relative importance of different materials properties such as viscosity, interfacial energy, and thermal expansion. In this study we focus on the effects of interfacial energy in composite microstructured fibers. We measure the interfacial energies at high temperature of a chalcogenide glass and an organic polymer recently employed in the fabrication of composite photonic bandgap optical fibers. We discuss the effect of interfacial energy during fiber draw, as well as the interplay between surface and viscous forces. Finally, we comment on the implications of this analysis for understanding what classes of materials can be used in composite microstructured fiber fabrication.