A method based on a controlled solid-solid reaction was used to fabricate heterostructures between single-walled carbon nanotubes (SWCNTs) and nanorods or particles of silicon carbide and transition metal carbides. Characterization by high-resolution transmission electron microscopy and electron diffraction indicates that the heterostructures have well-defined crystalline interfaces. The SWCNT/carbide interface, with a nanometer-scale area defined by the cross section of a SWCNT bundle or of a single nanotube, represents the smallest heterojunction that can be achieved using carbon nanotubes, and it can be expected to play an important role in the future fabrication of hybrid nanodevices.