Multi-walled carbon nanotubes (MWCNTs) with > 95% purity were synthesized over a Fe-Co/CaCO3 catalyst using chemical vapour deposition (CVD). Both the CNT yield and the outer diameters increased with time on line in the presence of acetylene. More significantly, the tubes were reduced in length and became stub-like with time. TEM analysis revealed that the CNTs commenced shortening after 2 h of reaction time. Reagent residues (e.g., Ca, CaO, OH/COOH groups and Fe-Co oxides) were found not to influence the CNT bond breaking reaction. CNT growth over Fe-Co supported on silica or CaCO3-Ca3(PO4)2 gave similar results. Further, MWCNTs produced by a floating catalyst method, carbon helices produced from Fe-Co-In/A2O3, and N doped CNTs also revealed tube shortening as a function of reaction time under a flow of acetylene. It is thus apparent that MWCNTs can readily be shortened by the facile procedure of depositing carbon from excess C2H2 on the outer walls of CNTs.