Single-molecule-like carbon nanocapsules derived from 12-filled ultrashort single-walled carbon nanotubes (US-tubes) have been prepared and characterized as a new paradigm in X-ray contrast-agent design. Functionalized by Bingel chemistry with external malonodiserinolamide (Ser) groups, the new I-2@US-tube (Ser), species are water-soluble (0.25 mg mL(-1)) and more than twice as X-ray opaque as their empty US-tube(Ser) precursors.