The efficiency of iron-catalysts in hydrocarbon decomposition, aimed at growth of carbon nanotubes by chemical vapour deposition (CVD), is systematically investigated. The synthesis reaction is carried out at different temperatures (873–1123 K), for various durations (0.5–6.0 h), using diverse precursor gases (C2H6 or C4H10) and catalyst supports (SiO2 or Al2O3). A large variety of experimental conditions is explored by varying amount (0.5–2.0 g), metal load (20 wt.% and 29 wt.%) and annealing temperature (723–973 K) of the catalysts and by considering different gas flowing setups, namely, by changing flow rate (100–240 cc/min) and composition (H2/precursor/He, with He at 0–63%) of the gas mixture, flow-rates and flow-ratio of reactant gases (H2: 0–120 cc/min; Precursor Gas: 15–120 cc/min; H2/PG: 0–3). Iron catalysts encapsulation is shown to be the main factor limiting C yields in the cases considered, and its changes to be responsible for the broad yield variations (20–910 wt.%) observed. The results of analyses, carried out by Raman spectroscopy (RS) and complementary diagnostics techniques, demonstrate the need of accurately tuning the manifold growth parameters, in order to fully benefit of the advantages potentially deriving from a proper choice of precursor gas and catalyst-support material.