Multiwalled carbon nanotubes (MWCNT) are synthesized by isobutane decomposition at 700 degrees C over as-purchased and iron-loaded K10-montmorillonite catalysts. The results show that, upon reduction at 500 degrees C, K10 catalyzes isobutane decomposition. Few carbon fibers accompany the prevailing carbon flakes formation. Upon Na+ exchange or by increasing the reduction temperature, the activity of the clay decreases. Fe-K10 behaves as a bifunctional catalyst: on added metal sites, MWCNT preferentially form, while on the support, carbon flake formation mainly occurs. At a given metal load, the increase of the reduction temperature up to 700 degrees C or the use of Na+-exchanged K10 as a support generally leads to an enhancement of the selectivity to MWCNT, because of the diminishing of the support active sites. Under the present reaction conditions, Fe supported on Na+ exchanged K10 are the most active among the investigated catalysts. MWCNT copiously form, both at low and high metal load, and exhibit the highest structural order.