The effects of varying concentrations of ferrocene and thiophene on the synthesis of carbon nanotubes (CNT) in a floating catalyst, flame-based reaction system were investigated. Pre-vapourised ethanol was used as CNT feedstock, and a surrounding premixed H2/air flames was used as the heat source. Samples of the synthesised material were collected and analysed using Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy to determine the properties of the collected CNT samples. Sulphur to iron mass ratios were varied from 0 up to 10.1, by changing the relative amounts of thiophene and ferrocene. The latter was found to have only a marginal effect on the improvement in the yield of CNTs, but led to enhanced yield of iron oxide nanoparticles. In comparison, the addition of sulphur exhibited an increase in the synthesis of CNTs as compared to iron oxide nanoparticles, including their number density and length. An optimum window of mass ratios mS/Fe at 0.1–2 was identified. A numerical model for the diffusion of species is coupled with an equilibrium chemical model for the resulting species to understand the mechanism of the synthesis process, showing the range of elemental proportion conditions and residence times favourable to high yield of CNTs.