Multiwalled carbon nanotubes (MWCNTs) have been synthesized by the floating catalyst method using toluene as the carbon source and mixtures of ferrocene and ferrocenyl sulfide as the catalyst. Products were characterized by both low-resolution and high-resolution (HR) TEM analysis. It was found that the presence of large amounts of sulfur in the reactant mixture generated only amorphous carbon while lower amounts of sulfur led to mixtures of MWCNTs and carbon fibres. The product distribution, yield and the tube diameters varied with the sulfur content. HRTEM analysis revealed that the MWCNTs were poorly graphitized. Comparison with data using other sulfur sources (S8, thiophene) suggested that the proximity of the sulfur to the Fe catalyst in the gas phase did play a role in the CNT formation and provides a method of ‘carrying’ elements to a catalytic site. Copyright © 2007 John Wiley & Sons, Ltd.