Sulfonation of linear alkylbenzenes (LAB) with sulfur trioxide gives rise to coloured by-products. The extent of colour formation depends on the quality of the feedstock and sulfonation conditions. Coloured species are thought to consist of polycyclic, polysulfonated aromatics, formed by oxidation, oligomerisation, sulfonation and cyclisation reactions. Colour formation is believed to arise by way of side reactions, the initial step being where sulfur trioxide acts as an oxidising agent possibly involving electrophilic attack on a C-H bond in LAB. Sulfur trioxide could abstract a hydride ion to yield a carbocation intermediate, which then loses a proton to give a double bond. This key step could lead to further reactions, which could result in increasing unsaturation, cyclisation, oligomerisation and polysulfonation, or a combination of these to form coloured by-products. Theoretically colour precursors should have C-H bonds that are susceptible to electrophilic attack. Compounds considered to be potential colour precursors are dialkylbenzenes, branched alkylbenzenes, diphenylalkanes and polycyclic aromatics all of which could have a tertiary C-H group that would be susceptible to electrophilic attack. A set of model compounds of these types, along with their unsaturated precursors were synthesised. When each of the model compounds was dosed into LAB and sulfonated, the result was an increase in the Klett colour of the resulting sulfonic acids. Further investigations revealed a linear relationship between concentrations of model dialkylbenzene and the model branched alkylbenzene and the Klett colour of the resulting sulfonic acids. A mixture of methyl branched alkylbenzenes was synthesised. When this mixture was dosed into LAB at varying concentrations, an increase in Klett colour was observed and a linear relationship was indicated. A set of model experiments was carried out involving reactions of some of the model colour precursors with DDQ. DDQ dehydrogenates hydroaromatic compounds following a similar route to that described above.