The metal carbides Li2C2, NaHC2, and CaC2, as well as acetylene, have been decomposed thermally and by metathesis reactions with selected transition-metal halogenides as catalyst precursors in the temperature range of 550–1000 °C. All reactions have been performed in selected salt fluxes which dissolve the ionic carbides and the transition-metal halogenides and which act as a thermal sink. While the decomposition reactions proceed at the upper temperature range, the metathesis reactions run well at the lower boundary, leading to a large variety of carbonaceous nanomaterials. Optimizations for carbon nanotube and nanofiber formation have been pursued, with the best yields resulting from the use of lithium carbide and acetylene precursors at the lower temperature limit. An influence of an external magnetic field on the carbon-fiber morphology was found. Highly magnetic iron colloids have been stabilized against air and acids by carbon coating through similar reaction routes. Specific magnetizations of up to 150 emu g–1 (1 emu g–1 = 1 A m2 kg–1) have been found when iron powder was added to the salt melt prior to the decomposition reactions.