Single-walled nanotubes (SWNTs) industrially produced and multiwalled nanotubes (MWNTs) we produced by catalysis were investigated by transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The observed spectral features, that is the peak position, intensity, and bandwidth, were related to some structural properties of the investigated samples. In such a way, information on the vibrational modes, nanotube diameters, and purity degree of the samples was obtained. The findings of the FTIR investigation are in perfect agreement with the TEM evidence.