Viscous, water-miscible, liquid multiwalled carbon nanotubes with an extraordinarily high degree of functionalization (hf-MWCNTs) have been obtained by means of a deep-sidewall modification process. The hf-MWCNTs behave as an ionic liquid, with a conductivity of 1 mS cm(-1) at 100 degrees C. The unique properties of this material render it suitable in a wide spectrum of applications, ranging from nanomedicine to energetics.