The work described in this thesis has been directed at the development of a novel synthetic route to alkylsalicylic acids. Associated reactions have also been studied. The primary aim has been the synthesising of alkylsalicylic acids possessing an alkyl chain containing more than eight carbon atoms. In addition, a limited study has also been carried out into the sulfurisation of alkylphenols. Both the alkylsalicylic acids and the sulfurised aJkylphenols are used as oil additives. They both act as detergents, keeping an engine clean and neutralising any acids formed in the engine as a result of oxidation processes. Chapter Icontains a general introduction to oil additives, principally the overbased detergents, and an introduction to Friedel-Crafts chemistry, which is the basic reaction employed in the alkylation of salicylic acid. Chapter 2 introduces the alkylation of salicylic acid employing concentrated sulfuric acid as the catalyst, and using simple model compounds to demonstrate the feasibility of the approach. The effect of varying the alkylating substrate to produce an alkylsalicylic acid with an alkyl chain containing at least eight carbon atoms is explored in Chapter 3. Optimization of the alkylation reaction and the effect of altering a number of the reaction parameters (e.g. temperature, catalyst and reaction duration) on the yield and product distribution for a range of alkylating substrates is set out in Chapter 4. The work contained in Chapter 5 concentrates on the synthesis and rearrangement of the esters of salicylic acid and investigates the possibility that the esters are intermediates in the alkylation reaction. Chapter 6 is concerned with the industrial implications of the alkylation of salicylic acid. It concentrates in particular on the synthesis using industrially available alkenes and the scale-up of the reaction. An insight into the sulfurisation of alkylphenols, and the attempted identification of products formed in the industrial process, can be found in Chapter 7. Finally, the experimental details for Chapters 2 to 7 are contained in Chapter 8.