High pressure and high temperature experiments were performed on single-wall carbon nanotube bundles up to 14.5 GPa and 1800 K. Depending on the thermodynamic conditions, we have observed three different behaviors: at ambient temperature and for pressure lower than 24 GPa, minor structural changes are observed. Depending on the loss of hydrostatic conditions or on the combined application of pressure and temperature, partial or total graphitization is observed. For pressures of 14.5 GPa and temperatures of 1800 K the nanotubes are irreversibly transformed into cubic diamond, showing that it is possible to synthesize under high pressure and high temperature pure sp3 carbon structures from single-wall carbon nanotubes. In the case of iodine intercalated nanotubes, the same conditions of 14.5 GPa and 1800 K lead also to the transformation into diamond. No evidence of incorporation of iodine in the sp3 carbon structure was found. On the basis of our results, we discuss possibilities for new carbon–carbon composite engineering from single-wall carbon nanotube bundles.