In this paper, the possibility of exploiting carbon nanotubes as X-ray waveguides is explored through analytical and numerical solutions of Maxwell's equations. It is shown that nanotubes coated with various materials can theoretically sustain electromagnetic modes. The factors determining the modes that can be established are the tube diameter and the field penetration depth in the cladding. The latter is shown to vary from 6.4 nm in carbon to 3 nm in metals such as gold and platinum. Numerical simulations confirm that the inner tubes in multi-walled carbon nanotubes have a negligible impact on mode formation, thereby, allowing a large range of tube diameters to be exploited. (C) 2003 Elsevier B.V. All rights reserved.