A novel thermogravimetric methodology is introduced to study the characterisation of coke precursors. The effect of different reactants, time on stream, and reaction temperature on coke precursor formation and chemical characterisation was investigated. This allowed us to classify coke precursors into "small" and "large" ones. Coke precursors are formed preferentially on the strongest acid sites at a rapid rate, with fast further transformation to other coke species. Furthermore, the method revealed a maximum of large coke precursors with time on stream due to their fast transformation into hard coke over strong acid sites compared with the much slower formation from small coke precursors over weak acid sites. (c) 2007 Elsevier Inc. All rights reserved.