A composition comprising a mixture of carbon nanotubes having a bi-modal size distribution are produced by reducing carbon oxides with a reducing agent in the presence of a catalyst. The resulting mixture of nanotubes include a primary population of multiwall carbon nanotubes having characteristic diameters greater than 40 nanometers, and a secondary population of what are apparently single wall nanotubes with characteristic diameters of less than 30 nanometers. The resulting mixture may also contain one or more other allotropes and morphologies of carbon in various proportions. The mixture of carbon nanotubes has specific apparently uncommon properties, including unusual resistivity and density