MgO-supported catalysts are used to produce high yield carbon nanotubes with few walls by the catalytic decomposition of acetylene. Low and high resolution transmission electron microscopy (TEM and HRTEM, respectively) reveals that the tubes possess many irregularities in their wall structure and that they appear as aggregates. The outer diameter of the multi-wall carbon nanotubes (MWNTs) obtained by this procedure is in the order of 5-10 nm while the inner diameter is less than 4 nm. Their resistance strongly depends on the post-synthetic treatments made upon them. Raw material obtained after synthesis is insensitive to ball-milling and tubes remain packed. The same feature appears after grinding the sample, treated with a concentrated acid in order to remove the support and the catalytic metal particles. The easy breaking of the tubes occurs after specific treatment Of the sample.